HP 500 backfire

[Geronimo36]

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]

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]

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]

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]

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]

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]

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]

Anytime! Lookin forward to hearing how you made out with the things we spoke about!

[Thunderstruck]

Any news??

[donzi2287]

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]

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

[MILD THUNDER]

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]

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]

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]

thunderstruck and mild thunder thank you for your advice. I'll post more as soon as I get more news on this backfire.

[donzi2287]

It is still backfiringIt 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.

[MOBILEMERCMAN]

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]

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]

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....re&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.

[MILD THUNDER]

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?