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1275 engine with SU HIF single carb

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RonR

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Hi Folks.

A friend is running a 1275 engine (bored to 1300 something) in a 1963 Morris "Mini," with the original donor car being unknown. The engine was rebuilt very recently by someone "back east." Currently, there is a brand new SU HIF 44 carburetor on the engine. It does not seem to run very well even after setting the fuel/air mixture using an exhaust gas analyzer.

The plug gap is set, the timing is set, and the points are set. We did not check the valve lifer clearances, rather assumed that the engine rebuilder did that properly. We did not check the float adjustment, since we would have had to remove the carb from the engine.

Initially, from spark plug readings, it was running way too rich and would only run for a few minutes. After resetting the carb to the initial "2-turns back’ from the jet being level with the bridge, it started and eventually ran for an extend period of time. However, only after the engine was warmed up would it idle under 1000 rpm. This was on a day where the ambient temperature was in the 90s. As I was tweaking the fuel/air mixture (using the exhaust gas analyzer as a guide) all of a sudden the mixture went super rich without anyone doing a thing to the car. We shut the engine off and considered what might have caused the jump in fuel. For whatever reason, we checked the compression and found it was between 150 and 155 psi on all 4 cylinders, so no problem there. So we tried to restart the car, and it would not start. So here I am, seeking the wisdom of the British Car Forum members.

Another friend, who restored British cars for over 40 years, said that some HIF carbs have a flapper thing on the butterfly that he would always solder shut to get the engine to run right. He also said for Minis, he preferred the original twin SU carbs over a single carb.

Any thoughts and suggestions would be appreciated.
Ron
 
Super rich means more fuel, less air - was there a blockage? I have heard of this soldering thing before though I can't offer an opinion about its usefulness - but, I don't feel like that is your problem.
 
air cleaner was off. No visible blockage.
When installed in a Morris/Mini, the engine, of course, is cross-ways, being a front-wheel drive vehicle. The carb is towards the firewall. But nothing is in the area the could have blocked the air flow
 
My understanding of the poppet valves is that they are part of the emissions system. One of my rovers (a 2000 SC) had the valve soldered shut, but it also had most
of the emission stuff removed, no air pump, etc. It almost sounds like there is an issue with the choke linkage, and the jet dropped out of position.
 
I'd pull the carb (don't know how big a pain that is on a Mini) and double check everything. I'm running a new HIF44 on the 1275 in my Morris Minor and it runs like a champ, no issues. Pulls strongly and idles well, so there's no reason it shouldn't work in your application.

I wouldn't trust the builder to have done the valves correctly either -- I admit I'm paranoid of any work I didn't do or see done -- and they're easy to check/set so that's worth doing. Likely not related, but simple enough to do so you'll know for certain.
 
How much fuel pressure are you running?? Could be fuel pump overrunning the float needle. Myself, i prefer the single carb to the dual's....less to tune and no synchronization needed. Specs on the HIF carbs are available and the only way to check fuel level is by draining the carb and checking against data. The main trouble spot on HIF carbs in my opinion is the temperature compensater bi-metallic strip arrangement on the main jet. The poppet valve on the butterfly can come apart on high mileage carbs and it, of course, also restricts air flow.

Kurt.
 
Since the engine will not start, tip a thimble of fuel down the carb throat and try starting again. If it fires and quickly dies, you have a fuel delivery problem. If it fires and continues to run, you probably have a choke adjustment issue. If it fails to fire you could have over-fueling from a leaking float valve or too high fuel pressure... or you could have an issue with the points closing up.

I don't know much about the HIF carbs but from memory, some of them have a filter screen on the float valve, some do not. There is always the possibility that you have one without the screen and that you have debris interfering with the float valve. I also remember that the direction you turn the jet adjusting screw on the HIF carb may be counter-intuitive. Be sure to look up the adjustment steps and that you are turning in the right direction. If you don't have specific instructions for the HIF carbs, that information will be on the Technical web pages on the Burlen web site.
 
Thanks for all the comments.
I will not be able to get to the car again until the weekend, but I will report back.

Ron
 
I think I'm with Doug on this....sounds suspiciously like a chunk of dirt suddenly got into the float needle to me.

Kurt.
 
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