Timing problem - backfiring - after supercharger install

Spent the day installing Moss Supercharger on my 1977 MGB, including the Petronix FlameThrower distributor. Friend who has lots of automotive experience and a garage/shop helped me with this project, however now my MGB is still in his garage because we cannot get the timing correct. After install we spent a few hours working on adjusting the timing, however continue to have the following problems: with timing advanced 13degrees, seems to idle fine, seems a little slugish when givein it more gas (like going from 900 rpm to 2200rpm) -- but the real problem is when we try to drive it just a few feet we get backfiring. Seems to be from the intake more often than the exhaust. We are doing this timing with the vacuum advance hose from the carb plugged. One we got the timming where it seemed good (checked #1 at top dead center at least 4 times and were right on) until we put the vacuum advance hose on the distributor...then it would sputter and backfire out the exhaust. We tried changing the timing advance to 10 degrees or lower but did not run well and got some backfiring. So we can get it to idle and we can rev up to 3000 rpm with no backfiring (as long as we are sitting in neutral). As soon as we go to reverse to back out of the driveway we get backfiring, like 1 or 2 pops from the engine (intake). Then it is hard to get it to move forward in 1st gear to get back in the garage because it is backfiring so much (from engine). Then once we are sitting in neutral it seems to run and rev up fine. Are we correct in keeping the vacuum advance hose from carb plugged and not attached to the vacuum advance on the distributor? Any thoughts on what we are missing? Seems like we have the firing order correct or is it possible to be off 180? So close.......otherwise it was a great day of installing a supercharger in my MGB.

Sounds like it may be running lean.


Dwight

Tom.

Check for intake manifold gasket leaks.

kelvin

Did you install it backwards <grin; just kidding>

I agree that it likely running lean. Did you set the mixture to the Moss specs.
I found them to be pretty darn close.

reduce the boost,

Obviously our car doesn't want a supercharger. It should be removed immediately and sent to me.

I also vote lean, I had that problem when I put my carbs back on and hadn't adjusted them at all. Most of the problem turned out to be the mixture, the rest was timing. If you've got timing down though it's probably just the mixture.

Obviously check for vacuum leaks, but also check the advance again.

I assume that the vac is ported at the throttle plate.

Use a dial back light while you run the revs to something near 3500 and look to make sure that all of the advance is in at about 35 degrees and that it gets there smoothly. That is what you should have on a normally aspirated engine. With a SC hanging there I suspect that you will want a few degrees more than that though. I am saying that while thinking that the more dense mix is going to need a tad longer to ignite and burn.

Jeff and others will have more first hand experience on this, and more thoughts on my theory, so wait until you hear from them. Kelvin works in the bowels of the beast so he might be able to check with the R&D guys who developed the kit. I suspect that they encountered similar problems while coming up with their published formula.

Don't try to run the car under load until you get this worked out. The combination of SC, wrong timing and improper mix is a sure fire recipe for piston damage.

Jack

Sounds familiar! You possibly have two issues - the first is that the piston is not rising properly in the carb. Moss put a restrictor in the vacuum port in the piston to prevent the piston rising too quickly. Unfortunately, this sometimes causes the piston to bind and lock so it does not rise. The cure is to drill out the port in tiny increments until it works properly - contact Moss and they will send you the instructions.

The second issue is ignition timing. I had a lot of trouble getting mine correct - and using a Flamethrower dizzy would never work! Send a Lucas dizzy to Jeff and tell him to curve it for the blower. You need lots of mechanical advance fairly early on I believe. Even then, mine didn't run properly through the rev-range until I disconnected the vacuum advance! Did that, set it up to idle smoothly and it works really well!

I think Kelvin is right - it sounds liek somethign is fundamentally wrong with the install, although I had thought they got the carb issue taken care of? Kelvin can probably add a bit of input to that issue and its current status?

Eric, I didn't know you had vac advance problems! I always surmised that vac advance should be limited with the supercharger, as it is with Weber DCOE carbs.

The Pertronix distributors have had more and more issues lately, and I've been consulting with them to help resolve the most common problems. It may be months before they make any changes however. They are NOT anywhere close to ideal for a supercharger. Double the vacuum advance you need for most applications, plus too much mechanical advance (at the wrong rate) for a supercharger. There's very little consistency in how the Flame Thrower distributors are produced, and that shows in the final testing. Lately (last 4-6 months), they come with play in the shaft bushings when brand new, so they act like a 30-40 year old unrebuilt Lucas distributor with a Pertronix installed. A compromise at best.

Thanks for all the advice. Back at work today, however talking with my friend/co-worker who is helping me with this project --- we believe in trying to set the timing we got messed up and were assuming rotation of rotor clockwise when it is counter-clockwise and may have messed up order of plug wires to distributor cap. Can someone send and isllustration of this? Will also start to adjust to see if we are too lean, as soon as we can get back to it.

#1 top right, #3 top left, #4 bottom left, #2 bottom roght as you look over the passenger fender at the top of the distributor cap.
An incorrect firing order WILL wreak havoc and cause the problems you describe!

Thanks to all. Now I just have to get out from work and try this....will update with results. Believe we know what we did wrong and will have it on the road soon. Fairly sure we have gotten the firing order messed up and will start with that, then also check to see if mixture is too lean. Then the fun begins!

One more question: for those who are running the Moss Supercharger and the FlameThrower distributor ---- how are you handling the vacuum advance? Are you connecting the line from the vacuum advance outlet on the supercharger to the advance unit on the exterior of the distributor or are you just plugging it on the supercharger and leaving things unattached to the vacuum advance on the distributor?

I just plugged the vacuum port at the blower.

Jeff - my apologies, I didn't realize I was having vacuum advance issues either, until the day I left it off and all the knocking went away! I am suspicious that the port on the blower gives too much vacuum too quickly (or all the time!) and that throws off the dizzy. Disconnecting it really was a "Eureka" moment - and since I have your dizzy on there, I never went back to the flamethrower - with which I had similar problems.

In general, I found getting the setup "just right" with the blower is a tricky business. Small changes in both mixture and timing can make huge differences - although once the ignition is sorted, the mixture changes are less sensitive.

FWIW - If I ever did it again, I would NOT install the blower on a newly rebuilt motor. I would run the engine on straight carbs until it was bedded in, and sorted - THEN I would install the blower. However, once it is running properly... WOO HOO!!!! - and who cares about horsepower - gimme the torque anytime!

All good advice. I had similar problems on my boat, worked great on the idle circuit, leaned out and backfired under load. Try to find a chassis dyno to fine tune your combination. At about $100 per hour, it is much cheaper than a rebuild. Also, you should run less timing under boost, 35 degrees sounds like too much to me, what does the Moss kit recommend?

30-33 degrees is about ideal for a supercharger. Unfortunately, if you run a distirbutor that's not built for the purpose, you need to push the upper limits to get it to run decent at lower rpms. The more timing you can get at idle the better. Its not unheard of to run 20 degrees at idle, 32 on the top end.

Tom. The Flamethrower distributor is supposed to have the same specs as the early model MGB distributor. The vacuum advance is supposed to be connected to the ported vacuum source on the HIF 44 carburetor, just above the butterfly.

It is definitely advantageous to hook up the vacuum advance! Eric, you will get improved cruising mileage if you hook the advance back up on your installation.

An interesting point, which Jeff can probably illuminate is that if you hook up the distributor to manifold vacuum (the fitting on the adapter between the carb and the blower), you will get additional advance at idle which tends to smooth out the idle and gives better tip in torque. This is a tip I got from Carl Heideman and it's how I have mine hooked up. It's not correct, but it worked on my car. YMMV.

kelvin

I find that "correct" is out the window when you add a supercharger to the B. I believe the problem Eric is encountering is too much vacuum advance, causing a backfire out the exhaust when under light load. WAY too much total timing, which has to be limited with the supercharger. That 20 crank degrees of advacne from a stock distributor (or from a Flame Thrower) is too much. I can tailor them down to any number if need be, or supply one designed for manifold vacuum to really work for this engine. I can't remember which unit Limey had. Maybe he can read the specs on it and chime back in? The 10-15-5 unit used in 73/4 is probably a good choice for this setup.

I was unable to work on the MGB tonight, however my friend did (it is in his garage) and once he changed the wires back around he sent me an email saying it is now running and he is able to drive it around the block without backfiring and idling is better. We plan on getting back to it Thursday night, will follow your advise and words-of-wisdom...and expect to be driving it home Thursday night after checking the timing again and the advance. Thanks again for all your help. Excited about driving it with the supercharger.

Not sure which unit is on the car - and I'm at work! However, if I set the car up to idle smoothly with the vacuum advance connected, I would have terrible knocking at anything over a very small throttle application. With the vacuum disconnected, and set for smooth idle, I only get knock if I floor it below 2000rpm! If I reconnect the vacuum at this point - everything goes to hell again!

I've always suspected that NO vacuum advance would yeild great power and performance, at the risk of a slight loss of fuel economy. Do you NEED fuel economy if you run the supercharger? About as much as when you run a DCOE race carb.

Tom,
I am glad to hear it was a simple problem.
Now you will be able to dial the engine in to get the most out of the blower.
It can be a bit of a frustrating experience but the end result makes it worth it.
One thing you get out of this exercise is a much greater knowledge of your
distributor and carb.
I went through the dialing in process once, then when I got it running sweet
I went and built a new engine. Now I am working my way through the tuning again.
I am a sucker for punishment, but I am looking forward to even better performance
once I get things sorted..........Again! :eyeroll:

Cheers,
Rich

Thanks for all the advice. My friend Karl, who has the garage, the tools, and skills did some fine tuning and I took it for a ride tonight....runs great....can't believe how much extra power I can feel....and how much smoother it runs (PO had put this 1977 back to dual SU carbs and a few other mods). Probably about 95% correct for mixture and timing, however I did notice when starting out in 1st gear, like from a complete stop, if I gave it too much gas---I got 1 backfire out the charger. This happened 4 out of 5 stops. Otherwise I was able to cruise at 55mph and it felt great. Since I am going on vacation Friday am for 6 days, I will not get a chance to drive it until next Friday when I get back....and Karl expects to 'play around with the timing" to get it right. Next dream.....5-speed transmission? Thanks again for all your help.....will update in 7 days when I get back and drive it for a few days....unfortunately Winter (snow) is coming soon and my MGB hibernates in my garage until March or April (I live near Green Bay,Wi.)

Tom, something as simple as a different spec vac unit may do the trick. I have just the ticket if you're interested.

Got a new vacuum advance unit from Jeff and installed today. Runs much better. Also moved it to manifold. Got rid of most of the backfire issue I had --only when starting from dead stop. Found it ran very good if I had it partially choked -- no backfire...but also just checked dashpot oil level and seems low...so will adjust that Sunday. After getting it running good it sure has lots of extra power and supercharger does make you smile more when you drive.

Only disappointing issue is both before and after the install of suprcharger -- I had (and still have) a problem with higher speeds. Can run forever at 55mph but if I go between 65-70mph after about 1/2 mile I lose power---feels like a sudden loss of any fuel in carb. I did replace fuel pump this past summer but that did not eliminate the problem. Now that I changed to supercharger, new single carb (instead of duals), new distributor, plugs, wires, etc....I was surprised to experience this same problem today. Thinking I may have to either replace the hard fuel lines and/or clean the gas tank this Spring. Winter in Wisconsin is here and time to put the MGB away for a few months.