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VetteLT193
02-16-2009, 02:46 PM
My brother has an HP 500. A little over 200 hours total run time on the engine. Roughly 20 hours of run time on new springs, cam, etc. (basic HP 500 rebuild). The engine is new to him and his boat.

He's got a Mighty Marine Demon 825 carb.
Black on the transom indicates a rich condition...
above 3500-ish RPM's there is an occasional backfire through the intake
It isn't 100% smooth over 3200 RPM's, you can hear it in the exhaust.

Where the heck should he start?

MOBILEMERCMAN
02-16-2009, 02:51 PM
I would start with a leak test. Or at least a compression test.

Geronimo36
02-16-2009, 03:00 PM
I agree with Jim... Plus, I've seen a lot of weird issues with the Demon's.

Might want to start with plugs and a coil if the transom is black.. Could also be something stuck in the needle and sead. Had a problem like that with a friends boat last year... It was both fuel and spark... seems things always happen in 2's or 3's. lol

VetteLT193
02-16-2009, 03:31 PM
I would start with a leak test. Or at least a compression test.

This was checked when the engine was rebuilt, it's been like this since he put it in the boat.

Plugs were just changed.
Brand new wires just came in, he's installing them soon.

I think it has to do with timing + something else. :ack2:

VetteLT193
02-16-2009, 03:37 PM
If it helps: When he initially timed it he said it ran like crap when he timed it to 34 degrees above 3000 RPM. That is what we were told to time it to. He backed it off to get it to run.

Anyone have ideas?

MOBILEMERCMAN
02-16-2009, 03:46 PM
What ever you do go at it systematically. Don't just throw parts at it. You can't assume anything. I start with compression or leak test. Followed by a load test in gear to eliminate a weak cylinder. Generally you find a clue by then. Fuel and ignition tests to follow.

Good luck

Griff
02-16-2009, 04:05 PM
Check timing to make sure its advancing properly. If its a stock HP module, set base at 14* and see if it advances correctly up to 3000rpms. It should not advance anymore after 3000rpms.

VetteLT193
02-16-2009, 04:06 PM
What ever you do go at it systematically. Don't just throw parts at it. You can't assume anything. I start with compression or leak test. Followed by a load test in gear to eliminate a weak cylinder. Generally you find a clue by then. Fuel and ignition tests to follow.

Good luck

He hasn't thrown a part at it that it didn't at least probably need. Plugs and wires were simply maintenance stuff, he had initially re-used them off his old engine.

Any comment on the timing? if it ran like crap from day 1 with the timing at 34 degrees above 3000 does that mean something? I have a feeling the cam timing is off. I don't know where it is, purely an educated guess at this point.

Geronimo36
02-16-2009, 05:24 PM
Any comment on the timing? if it ran like crap from day 1 with the timing at 34 degrees above 3000 does that mean something? I have a feeling the cam timing is off. I don't know where it is, purely an educated guess at this point.

It's kind of hard to say exactly what it is without hearing it or being there..

If cam timing was off it probably wouldn't run right at all.

But, it can also be the ign. module. The modules were known for going bad, especially the older plug-in modules that had the module bolted to a plate instead of on the side of the dist. Sometimes it's even the connection on the module.. What a PITA those problems were to fix!

Could also be a coil. At lower RPMS it works and at higher RPM's it breaks up. Those older HP500 coils had a nack for cracking and causing an arc to the negative side of the coil... Under say 3000 rpms the motor sounded fine.

So many things to check... just throwing a couple things out there.

flenner
02-16-2009, 09:00 PM
Check the Module. Happened to one of my 500 HP's last year. Was doing the same thing that you are discribing. I changed plugs, wires, distributor cap/rotor, distributor pick up, etc until I figured it out. It needed a tune up anyways.

Thunderstruck
02-16-2009, 09:16 PM
Dist cap and rotor.

New Coil? Make sure it doesn't have an internal resistor. Use a Merc coil or a MSD high vibration coil. The oil filled coils tear up laying on their side in a stock 500HP carb bracket. Replace the coil if you haven't. A coil with the internal resistor will cut voltage to coil and reduce the firing voltage. Usually shows up at higher rpm since the lower voltage will run OK at idle/low rpms.

Run the new wires correctly.

Fuel pressure gauge?? If none install an auto part cheapo temporarily with a long hose to watch while running. Need 6-8 psig all the time. Bad fuel pump, clogged filters, will make it run lean and will pop out the exhaust.

Was the cam changed?? Post the cam card on here and someone will help. New or not, was it installed straight up or degreed toward the retard direction?

Use a better timing light. The Sears $80 one works well. Bad timing light = bad timing. Move the location where the timing light is hooked to #1 to eliminate cross firing. You did hook it to #1???

Distributor installed correctly? This is my first choice since the timing had to be retarded to get it to run.


Float level in the secondary fuel bowl at proper level?

Voltage on the ignition wire OK? Run a hot wire to the + side of the coil directly from the battery and run it under load. (After changing coil.)

VetteLT193
02-16-2009, 09:36 PM
All good stuff to look at, thanks!!!!

The cam is the updated/better cam, 731 rings a bell?

any more ideas, please post...

we'll post updates as things check out. My brother is not too computer savvy so I post for him a bit :boxing_smiley:

Raylar
02-19-2009, 02:32 AM
The compression test is also a really good palce to start as if the valves are for some reason adjusted wrong with them hanging open a bit at higher rpms a backfire and popping is quite common. The compression test will tend to eliminate a lot of physical problems and this should narrow your search if no physical problems show to the spark and carb issues.
Its really hard to chase to many possibilities at one time, narrow the field.

Best Regards,
Ray @ Raylar

donzi2287
02-20-2009, 08:03 PM
O.k. compression check as follows: 1/168, 2/168, 3/170, 4/ 174, 5/ 170, 6/ 170, 7/ 170, 8/ 150. The number 8 plug looks fouled. Also, the spark on all plugs looks orange and not blue in color???? Any suggestions?:ack2:

Griff
02-21-2009, 03:49 AM
#8 might have an intake valve hanging open a little bit. I wouldn't worry about the spark color.

Raylar
02-22-2009, 02:14 AM
Squirt a little compressed air in #8 cylinder with that cylinder at TDC or both intake and exhaust valves backed off or definitly positioned for both closed and listen carefully where the air is escaping. If its coming out the intake under the carb, you've either got an intake valve on that cylinder hanging open possibly from a incorrect adjustment or a slightly bent or sticking valve or a bad valve spring. This will definitly create a pop out the carb under load. Might be your issue.

Best Regards,
Ray @ Raylar

donzi2287
02-23-2009, 11:07 AM
The # 8 exhaust valve was tight, so I loosened it 1/2 turn and it seems right. It still backfired after I opened it to full throttle then slowed down to @ 3000 rpm. I'm going to change out the coil next as the one I got is from NAPA and I think it is just a basic one. As far as the carp it is one and a quarter out in the center screw. Other than that I am lost!!! Any suggestions on tuning it ?? Thanks again.

MOBILEMERCMAN
02-23-2009, 11:14 AM
Make sure it is a non resistor coil

MILD THUNDER
02-23-2009, 10:27 PM
The # 8 exhaust valve was tight, so I loosened it 1/2 turn and it seems right. It still backfired after I opened it to full throttle then slowed down to @ 3000 rpm. I'm going to change out the coil next as the one I got is from NAPA and I think it is just a basic one. As far as the carp it is one and a quarter out in the center screw. Other than that I am lost!!! Any suggestions on tuning it ?? Thanks again.

Center screw? Which screw are you adjusting? There should be idle mixture screws on each side of the carbs, however, they are just that, idle mixture screws. When at 3000+rpm, there setting should be irrelevent to the issue. I have seen where a engine can have good numbers on a compression test, but have a flat cam lobe, or other valvetrain issues.

Are you 100% that all the valves are adjusted properly? This is critical. If they are, I would look into making sure the front and rear floats are adjusted properly, and that the powervalves are not blown and are of correct size. If it seems to run well at WOT, but bad in the midrange and low end, i would look at the powervalves.

tunnelvision69
02-24-2009, 12:33 AM
to backfire through the carb one of 2 things needs to happen. 1) intake valve not closed (broken spring, overtightened adjuster) etc. or 2) ignition at the wrong time (arcing dist cap,bad module or voltage crossover) I have seen voltage jump from one plugwire to another that runs parallel to it. check plugwire routing & make sure the wires don't run next to each other especially if they follow each other in the firing order. I have seen this severe enough to cause engine damage due to detonation.

Geronimo36
02-24-2009, 10:18 AM
to backfire through the carb one of 2 things needs to happen. 1) intake valve not closed (broken spring, overtightened adjuster) etc. or 2) ignition at the wrong time (arcing dist cap,bad module or voltage crossover) I have seen voltage jump from one plugwire to another that runs parallel to it. check plugwire routing & make sure the wires don't run next to each other especially if they follow each other in the firing order. I have seen this severe enough to cause engine damage due to detonation.

#3: Lean - if the engine is overly lean it will pop back thru the carb as well.

Geronimo36
02-24-2009, 10:20 AM
Center screw? Which screw are you adjusting? There should be idle mixture screws on each side of the carbs, however, they are just that, idle mixture screws.

He has a Demon, they have an extra adjustment screw (Idle Eze) in the center of the carb where the flame arrestor bolts up to.

Geronimo36
02-24-2009, 10:24 AM
The # 8 exhaust valve was tight, so I loosened it 1/2 turn and it seems right. It still backfired after I opened it to full throttle then slowed down to @ 3000 rpm. I'm going to change out the coil next as the one I got is from NAPA and I think it is just a basic one. As far as the carp it is one and a quarter out in the center screw. Other than that I am lost!!! Any suggestions on tuning it ?? Thanks again.


To do it right you really need to re-adjust the valve from scratch, not just loosen it up a bit. I dont' think this is your problem but since you checked it out, may as well do it right. Turn the engine over until the valve is all the way closed and the same valve on that cylinder is depressed. Loosen the rocker arm, then slowly re-tighten it while spinning the pushrod with two fingers. Once you feel tension on the pushrod, tighen the nut a 1/2 to 3/4 turn. Do both the intake and exhaust on that cylinder to be sure.

Let us know what you find once the coil is replaced.

Airpacker
02-24-2009, 10:44 AM
make sure #5 and #7 plug wires are not crossed. You say the cam was swapped, make sure its not a firing order swap profile as well.

donzi2287
02-24-2009, 06:48 PM
Just to add the whole top end is new. Also it seems to hesitate/ run rough at 1300 to 1600 rpm. The center screw dead in the middle top of carb. Another note is the screws on the side of the carb in the front seem to be out a lot. Once I turn them in, it doesn't like to idle. So now what???? I am going to pull both valve covers off and check the adjustment of the valves.

Airpacker
02-25-2009, 09:28 AM
you need to get a vacum guage on that thing and check manifold vac at idle and during the poor running condition. That will tell you alot about cam timing, ign timing etc.

donzi2287
02-25-2009, 09:35 PM
Thank you to all and thanks to Geronimo for the phone call!! I'll update soon. This is frustrating!!!! One more new add is after Airpackers note.... I don't have that little thing on the back of the manifold in front of the distributer feeding to the back side of the manifold below the carb. I put a plug on that hole.

Geronimo36
03-01-2009, 04:28 PM
Anytime! Lookin forward to hearing how you made out with the things we spoke about!

Thunderstruck
03-02-2009, 05:34 PM
Any news??

donzi2287
03-03-2009, 10:33 PM
O.K. Here's the new poop.....This is frustrating as I'm scratching my head cursing with a beer in my hand! I have adjusted the intake and exhaust valve on #8 cylinder and ending up loosing @ 1/2 turn on both. I installed a new plug in that same cylinder, just because I was trying to make all equal. By the way, the plug after 2 hours looked a little burnt on one side of the little white thing!!! So, It runs the same. It is still popping on that side of the exhaust. I did do WOP for just a second, literally, and it didn't backfire, but I now know if I stayed on it it would have. Basically, I have no improvements and I keep throwing my time and money away. Thank you so much Geronimo for our phone conversation. I think it would cost me less to fly you and your wife here, wine and dine you and you would find the problem in 5 minutes!!! I would be ahead of the game. Anyway, It is still popping/ misfiring on that side of the exhaust.

Thunderstruck
03-03-2009, 11:05 PM
Could you list everything you have done to correct/diagnose the problem? Interesting problem.

MILD THUNDER
03-04-2009, 09:57 AM
Now its backfiring thru the exhaust? I see that you posted twice that when adjusting the valves, you had to loosen them? When you adjusted them, did you follow geronimos valve adjustment procedure? it is mandatory that the valve you are adjusting has the lifter on the flat side of the cam lobe.

Rotate the crankshaft until the pulley notch aligns with the zero mark on the timing tab. This positions the No. 1 cylinder at TDC..
THIS CAN BE VERIFIED BY PLACING A FINGER ON THE NUMBER 1 ROCKER ARMS AS THE PULLEY NOTCH NEARS THE ZERO MARK.iF THE VALVES ARE MOVING,THE ENGINE IS IN THE NUMBER 6 FIRING POSITION:ROTATE THE CRANKSHAFT PULLEY 1 FULL TURN TO REACH THE NUMBER 1 FIRING POSITION.aNOTHER WAY IS TO TAP THE ENGINE AROUND WHILE HOLDING YOUR FINGER IN THE NUMBER 1 SPARK PLUG HOLE UNTIL YOU FEEL COMPRESSION....yOU JUST NEED TO MAKE SURE YOUR ON THE COMPRESSION STROKE.

With the engine in the No. 1 firing position,adjust the following valves.

Intake: 1,2,5 and 7
Exhaust: 1,3,4 and 8
To adjust each valve,make sure that the pushrod is properly centered in the rocker arm socket,then tighten the 9/16 locknut on the rocker arm until there is no lash(zero clearance) between the valve stem and the rocker arm.Note: Without lash,the pushrod will not turn.Then tighten the locknut (the book calls for 1 turn after 0 lash)I usually tighten them 1/2-3/4 of a turn after zero lash .Then tighten the allen screw while holding the locknut.

Rotate the crankshaft one full turn to realign the pulley notch and the timing tab zero mark in the #6 firing position,and then adjust the following valves:

Intake:3,4,6 and 8
Exhaust: 2,5,6 and 7.

Geronimo36
03-04-2009, 12:15 PM
Thank you so much Geronimo for our phone conversation. I think it would cost me less to fly you and your wife here, wine and dine you and you would find the problem in 5 minutes!!! I would be ahead of the game. Anyway, It is still popping/ misfiring on that side of the exhaust.

No problem. If I were in FL I'd stop by and take a look for ya. BTW, did you put in the MSD coil or replace the one you had? Also, did you disconnect the tachometer lead on the coil to eliminate a bad tach? What's your compression numbers now that you've adjusted the valve? If you could post some detailed pictures of the plugs that might be helpful as well. They should be white which a slight tan ring on the tip of the ceramic electrode. Did you have the Demon running on this engine prior to the top end rebuild? Lastly, do you have a ground wire running from your ignition module bracket to the engine? Sorry I'm not there, checking the basics would definitely be easier if I could see/hear it.


Could you list everything you have done to correct/diagnose the problem? Interesting problem.

I agree, put together a list of changes and in what order. Gotta take a systematic approach for stuff like this.

donzi2287
03-05-2009, 12:30 AM
I have put the new MSD blaster coil in. I didn't do another compression check after I adjusted the valve. The Demon was new, but it has been on my 454 twice for @ 7 hours each time not realizing my oil pick up wasn't right....spun bearings each time. It has been played with too many times with too many mechanics. I think I'm going to send it back to Demon to reset it back to factory specs. I will post a pic of the plug, it is firing on one side of it. Geronimo, Don't you want to leave the cold? There is a classic show in Mt.Dora (close to Orlando,Fl.) at the end of this month!!!!

donzi2287
03-05-2009, 12:33 AM
thunderstruck and mild thunder thank you for your advice. I'll post more as soon as I get more news on this backfire.

donzi2287
03-10-2009, 10:36 PM
It is still backfiring:sifone:It seems to happen now after 30 minutes running time and after 3000 rpm plus. the backfire is every 3-7 minutes running over 3000rpm. Now, seems to be worse at high end too.....I don't get full rpm and it is very rough sounding. Help I can't take this anymore.:leaving:

MOBILEMERCMAN
03-11-2009, 12:28 AM
Sadly it might be time to comp check again. Better yet do a leak test. The valve that was tight may be burnt now. Also pressure check and/or inspect exhaust for water.

It may be more difficult to do a leak test , but it tells you so much. Good and bad news.

donzi2287
03-11-2009, 07:29 PM
I hope it's not a burnt valve.... I will do a leak down and compression this weekend and see what happens. Thanks again.

Geronimo36
03-14-2009, 09:25 AM
I hope it's not a burnt valve.... I will do a leak down and compression this weekend and see what happens. Thanks again.

Yeah, better check that again and let us know what you find.

Couple things;
1. did you disconnect the grey tachometer wire from the coil and do a test run?
2. have you tried a new ignition module?
3. Make sure the ignition module is properly grounded.
4. Sounds silly but check the valve cover and engine for oil leaks that might drip down onto the wires.
5. since it's running rough, get a timing light and check each wire, including the coil wire, to make sure they're firing steadily. Then with the engine running, use a pair of wire boot pliers (rubber insulated) one at a time pull a wire off the top of the distributor cap. When you pull the wire, take notice if the engine runs rough or the same. If it runs rougher put the wire back on the cap and move to the next one. If you pull a wire and it doesn't change the roughness of the engine you know you have a problem on that particular cylinder.
6. You may also want to pull the bowls off the carburetor and make sure a metal shaving hasn't gotten in the bowl or stuck in the needle and seat.

Here's a link for what the wire boot pliers look like, they should have some cheap ones at any local auto store. http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID=78286&group_ID=1351&store=snapon-store&dir=catalog

Just a couple things to check.... This list is long but just throwin some stuff at ya to eliminate the stupid stuff first. :ack2:

MILD THUNDER
03-14-2009, 01:10 PM
If it were a burnt valve, it wouldnt be a intermittant issue. Its sounding ignition related to me, since its a intermittant problem. I would double check all connections, and make sure you have 12v at the coil. Could be a bad pickup, or module.

Also, when you replaced the cap and rotor, did you put locktite on the rotor to keep it from coming loose? And your sure there is no moisture in the cap?

donzi2287
03-17-2009, 12:31 PM
You won't believe it. When I loosened the exhaust and intake valve, I also leaned the carb because I was way too rich. Well, I over leaned it. I richened it back to where it supposed to be, and it is running perfect. The compression came up on 8 cylinder as well. All cylinders are within 8 pounds!! That was an easy fix but also a pain!!!

Geronimo36
03-17-2009, 05:15 PM
You won't believe it. When I loosened the exhaust and intake valve, I also leaned the carb because I was way too rich. Well, I over leaned it. I richened it back to where it supposed to be, and it is running perfect. The compression came up on 8 cylinder as well. All cylinders are within 8 pounds!! That was an easy fix but also a pain!!!

Glad you figured it out!! :26:

It was showing rich cause that cylinder was bleeding off... Just goes to show that you can only make one change at a time and always start with the stupid stuff first! :blush5:

Frank

Thunderstruck
03-18-2009, 08:50 PM
Great job, thanks for sharing.

donzi2287
03-18-2009, 09:28 PM
Thanks again! You are right....one thing at a time!!!! Eddie

MOBILEMERCMAN
03-18-2009, 10:19 PM
Glad you figured it out!! :26:


Frank

It is nice to hear good news.:)