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MG MGB Technical - Help! HIF issues.

I have converted my '77 B to HIF's and points distibutor. The carbs had been rebuilt by another person in the past and shelved. I refreshed them with a few new gaskets/seals, check float level, etc. and new throttle shafts with solid throttle plates from Joe Curto. I've done this rebuild in the past with no problems.

The issues are:
Backfiring through front carb and acts like it's starving for fuel on acceleration, rough idle.

I have verified dwell and timing (10 BTC). Good reading from vacuum gauge, very slight needle movement. Good delivery at carbs from fuel pump, fuel filter fairly new.

Any ideas would be most appreciated!
David Steverson

PS I had read the other thread "HIF carb woe" and failed to mention:

I too have the issue with raw fuel coming out of the vent pipe on occassion. My car is missing the carbon canister at the moment. I tried pluging off that vent line this morning, maybe a bad idea....

The heat shield is in place and has good insulation.

My fuel pump is stock SU pump, not new but working well.
David Steverson

Plugging the vent line forces the fuel to go into the engine (It has to go somewhere, right?), causing flooding.
Steve S.

Thanks Steve! Elementary stuff I should have realized, just being an "eejit" this morning.

I took that plug off and idle is <b>much</b> better, but it's still starving for gas around 3000 rpm.
David Steverson

Backfiring through front carb with rough idle and poor acceleration - ignition and/or lean mixture problem (cylinders 1 and/or 2). Give us more history. How was it running before this conversion and how long ago? With timing light hooked up - is your timing advance (mechanical) smooth and steady?

Other things you might want to check:
1) Did you put plug wires back on correctly? Firing order is 1-3-4-2. #1 plug wire is at 1 o'clock position on distributor cap and rotor button turns counterclockwise.

2) Use your timing light to verify that all plugs are firing consistently. Your might have a bad plug or wire. Might even be a bad cap or rotor button.

3) Verify that the front carb piston moves freely within its air chamber (not running) and mimics piston movement of back carb while running in your driveway (idle, slow and quick rev up). Warning - don't get you face too close to that front carb.

4) Check cylinder compression. Fuel/air ignition with the intake valve still open (stuck) will blow out through the carb.

5) Check your intake manifold. Is it snugged up properly to the head? Make sure you haven't lost the front end plug (shaped like a little freeze plug).

Steve Buchina

Steve (feom AL), I've now checked all that and found no issues.
I've gone back over some bits and bobs and have all but one issue resolved: the fluttering or seeming to run out of gas around 3000 rpm.

It's idling well now and accelerating well until that point....
I did check the old floats for leaks and made sure the level was set correctly. I wonder if I have an issue with jets or metering needles. They all looked OK and I did follow the Bentley manual to make sure the needles were placed correctly.
David Steverson

David, What weight oil are you using in the carb dashpots? I ran too lite an oil and had a similar reaction. RAY

Not sure Ray, I have some in a little pump can, I thought it was 20-50 but maybe not. I'll try that. Thanks!
David Steverson

It's good that the backfiring has stopped. Not changing (doing) anything to resolve suggests that it may have been a sticky valve.

Ray's suggestion is a good one. That's why I was wondering if your pistons were in sync (e.g., relative height from idle to say 4,000 rpm, slow and quick throttle bumps).

You're using a new (rebuilt?) 45D4 distributor. Where did you get it? It's been said that some (many?) of these have poor/incorrect mechanical advance profiles (thinking weights and springs were the culprit). How many degrees advanced are you seeing from say 1,500 rpm to 4,000 rpm?

Bad thing about the HIF carbs (and my preference for the HS4's) is you can't open up the float bowls and compare fuel levels. You might be having a fuel shortage problem with the front carb (or both), but because I'm not all that familiar with HIF's, others will have to chime in and share their experience on how best to evaluate.

Have faith and enjoy the puzzle. With input from others, you'll get it running proper.

Steve Buchina

The distributor is a new one from BHive in South Carolina. I've not checked the advance with my light. I'll do that too.

I often think how great a site this is and how we all help each other out. I wish the same was true for my Subaru!

David Steverson

Well, the damper oil wasn't the problem.

Previously, the car had a Weber and ran fine. So, I don't think it's engine mechanical problems.

Here's where I am: the car idles great and accelerates fine up until the 2500-3000 rpm range.

Then it stumbles, flutters, acts like it's short of gas and I get the occassional backfire through the front carburator.

I must have a carb. problem or an ignition problem. I've checked and rechecked all kinds of things, no joy.

Murphy's law strikes again!
David Steverson

Few more thoughts and questions:

Was the new distributor installed before you changed to the HIF's (i.e., worked fine with the Weber) or were they installed at the same time?

Have you disconnected your fuel feed lines and checked (compared) flow rate from each? Pipe feed to front carb may have some blockage. Would seem unlikely, but I once pulled a split HIF fuel tube from the rafters in my garage and found the front portion block with some bug casings. You might also try filling your float bowls before starting (i.e., switch on until pump stops clicking), then start and quickly run up to 3,000 rpm. If engine runs well for some time before it begins to flutter, then you might have a float adjustment problem. Run this test with the engine already hot.

Have you checked cylinder compressions? I'm still thinking about the possibility of a sticky (valve guide) or poorly seated (carbon build-up) intake valve. A leak down test might prove useful.

If you don't have your own spare distributor, could you swap out with a friend who has a good running B? It's one of the fast ways to confirm or eliminate a problem with your own distributor.

Steve Buchina

I installed the distributor and carbs in one go.

I'll try that float bowl test.

You know, the floats, jets and metering needles "looked" OK when I had the carbs apart. I'm wondering if I should spent the money to buy those parts new.

David Steverson

I had this problem when I converted from HIFs to HS4s on my 76. (because of the overflow problem). Any air leaks can cause it, manifold, cyl head, or even the engine vent pipe split. But mine was simply balance between the carbs, with mixture rich on the second carb, which gave it a good tickover, and very weak on the first which caused a misfire when accelerating. I used a gunsons colourtune to set up the mixture on each carb with every nut on the the spindle loose to make sure there wasn't any influence from the other carb. You can then balance the carbs, I was lucky in that I have Crypton synchro tune, but a hosepipe and an ear will do.
c cummins

"Plugging the vent line forces the fuel to go into the engine" It's worse than that, not just intermittent overflow goes into the engine, it stops the float rising and cutting off the fuel, so the pump continually pumps fuel up the jets and into the inlet manifold!
Paul Hunt

I have the same issue on HIF carbs on a Jaguar Xj6 Series 2. Was driving fine one day and not the next. The rear carb had not vacum, plugs on cylinders 1 and 2 badly fouled after a short run. Have improved the idle, balanced the carbs, fixed air leaks, replaced needles ans floats. By-passed AED. Revs well and up to 3000rpm, but over this the carbs flutter and then the car stalls. Help..
V Harris

Are you sure the floats are not installed upside down. ie reversed. I had this problem about a year ago and it is surprising that the car will run fairly well in that state. Mine intermittantly lost power on the open road.
Ian Buckley

This thread was discussed between 13/07/2008 and 05/02/2012

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