HP 500 backfire

[VetteLT193]

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]

I would start with a leak test. Or at least a compression test.

[Geronimo36]

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]

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.

[VetteLT193]

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]

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]

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]

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]

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]

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]

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]

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

[Raylar]

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]

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?

[Griff]

#8 might have an intake valve hanging open a little bit. I wouldn't worry about the spark color.

[Raylar]

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]

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]

Make sure it is a non resistor coil

[MILD THUNDER]

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]

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.