MTFCA Forum

Hi all looking for some help.
I am working on a speedster in australia that has a backfire issue.

It is running a stipe 280 camshaft with 7.5 degree advanced cam gear, frontenac head high compression piston and distributor.

I have tried with both zenith carby and su carby.

Its issue is it splutters and backfires changing ignition timing doesnt make much difference putting the su carby helped a bit but not much.
If you load it up it is worse.

Thanks in advance for any help

Ian

Check the specs of the cam you have, you might be doubling up on the advance with the 7-1/2 gear as well.

I once read so where excessive carbon buildup could result in backfiring. The carbon heats up to the point of glowing and pre ignites the fuel in the cylinder before the intake valve is completely closed. Is there anything to this scenario? Has anyone else ever heard of or experienced this? I haven’t, personally, but it make sense. Jim Patrick

ian schmidt wrote: ↑

Wed Aug 18, 2021 6:09 am

Hi all looking for some help.
I am working on a speedster in australia that has a backfire issue.

It is running a stipe 280 camshaft with 7.5 degree advanced cam gear, frontenac head high compression piston and distributor.

I have tried with both zenith carby and su carby.

Its issue is it splutters and backfires changing ignition timing doesnt make much difference putting the su carby helped a bit but not much.
If you load it up it is worse.

Thanks in advance for any help

Ian

Ian,

Just so we understand, when you say "backfire", do you mean firing back out of the carburettor, or an explosion in the exhaust pipe/muffler?

Determining whether or not a Stipe cam also benefits from an advanced timing gear, (as mentioned above), is a good start. Otherwise, I think you're getting a rich mixture that's not completely consumed in the comustion chamber. Maybe running "too much carburettor"? Need smaller jets? Obstruction in the intake passages causing atomized fuel to recondense before entering the combustion chamber, (happened to me)? Vacuum leak that has caused you to set your carby way too rich in order to compensate? Bad mojo?

Sorry should have stated backfire mainly exhaust but also carby at times.

If i lean the carby out until it starts to die it still does it.

Seems it doesnt matter rich or lean it still back fires.

I have been thinking its the 7.5 degree timing gear.
But others say it is ok.
Wondering if its the combination of all of the mods.
Also has new plugs,leads, points and condensor

Ian

Ignition timing and advance curve could be a factor. Too much cam and or carburetion to run well at lower speeds could be, too.

Back firing whall running or after you shut it off?

It sounds like it is taking in more air than it can efficiently get rid of, ie; back pressure, has the exhaust been modified? If not maybe it needs to be to get rid of the exhaust and therefor the back fire.

When you checked the distributor timing did you check or replace the points and condenser ? A condenser going bad can cause a backfire, worn points can cause a erratic and ruff running engine. If you have a solid state distributor check for a bad diode. If new points and condenser then check for a bad coil wire intermitly grounding the coil or causing a open which would make the coil fire at wrong times.

Dennis Prince wrote: ↑

Wed Aug 18, 2021 9:04 pm

It sounds like it is taking in more air than it can efficiently get rid of, ie; back pressure, has the exhaust been modified? If not maybe it needs to be to get rid of the exhaust and therefor the back fire.

HI only extractor and open muffler.

Do you have a ground wire connecting distributor to engine block? If not add one. Its cheap and easy and will rule out intermittent open ground.

ian schmidt wrote: ↑

Wed Aug 18, 2021 5:21 pm

I have been thinking its the 7.5 degree timing gear.
But others say it is ok.


Ian

It only matters what Bill Stipe says. I'd ask him.

Hi Ian,
Has this back fire issue just come up or is this a fresh build and just getting going? Other than the back fire how does it run?
Should be pretty impressive. What dissy are you running, I had a simular issue with a engine I outfitted with a Thomas on the run stand. Come to find out the point cam was not concentric. Two lobses opened the points correctly the others where one to little & the other to wide. so the dwel was all over the place. So the timing was off on two cylinders. Swapped out the dissy & alls fine. Just a thought.
Craig.

Some times Ockhams razor is the best tool in the box.

Craig Leach wrote: ↑

Thu Aug 19, 2021 6:03 pm

Hi Ian,
Has this back fire issue just come up or is this a fresh build and just getting going? Other than the back fire how does it run?
Should be pretty impressive. What dissy are you running, I had a simular issue with a engine I outfitted with a Thomas on the run stand. Come to find out the point cam was not concentric. Two lobses opened the points correctly the others where one to little & the other to wide. so the dwel was all over the place. So the timing was off on two cylinders. Swapped out the dissy & alls fine. Just a thought.
Craig.

Some times Ockhams razor is the best tool in the box.

What you observed may, or may not, have been intentional…

Thinking way back to my VW days, if I remember correctly, most of the air-cooled VW distributors had a funny thing about their timing:
(from landracing.com forum) “ And a VW high performance enthusiast...Most VW distributors that I know of, had the lobe for #3 cylinder retarded because of the fact its cooling air passed through the oil cooler before cooling the cylinder...retarding the cylinder timing sorta/kinda/was supposed to help keep the cylinder within reasonable cooling specs...Well, in practical applications (meaning every air cooled VW I know of) it didn't work, #3 cylinder was famous for dropping its exhaust valve.”

Just something to be aware of for modern conversion distributors…

Eric

I’m with Kerry. I started to install a 71/2 deg gear but I checked with Dave at Chaffin’s about there 280 cam. It’s already advanced. I asked if they were marked so and he said no but that would be a good idea. I’m with Kerry. Check the cam

I ran a stipe 280 for years with a 7.5 advanced cam with no issues other than it ran rough from idle to about 1,000 rpm. Then smoothed out and would run very well with a good power curve after that. But the low rpm was not good. Required a lot of clutch slipping.

Eric.
I think I see the confusion here. Please don't take offence I thought it was very ironic that a distributer built a year before Ferdinand Porsche designed the Volkswagen would get confused.
Craig

Ok problem appears to be fixed.

After trying another standard timing gear although it helped it didnt fix the problem above idle.

So back to doing more tests checking everything from plugs to compression to valve timing and ignition timing all again.

Everything checking out to be right.

Still same issue

Out of just trying stuff i replaced the new coil with a old one i had laying around.

Start the motor with the zenith carby back on and low and behold runs like a dream with no backfire .

so i refitted the 7 degree cam gear restarted and we have a t model motor on steriods.

So now to finish the speedster off.

Fantastic good to hear. Thanks for letting us share our experiences. I guess Ockhams was right. G'DAY.
Craig.

Excellent! It's usually something very simple.... that's very difficult to find .

I had an 8N Ford tractor that ran like new for about half an hour or so, then it would begin to miss intermittently. After much fiddling, I found that it was the coil.