MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - HIF4 Tuning Question

Newly rebuilt HIF4 carburetors on my 74.5 BGT with a new heatshield.

Runs GREAT except for a minor issue I'm trying to figure out.

I have the carbs balanced and the idle mixture set properly. (Even the lifting pin trick works and I've never seen that before.)

Idle speed is rock-steady at 850rpm BUT after I've been driving for about an hour - which I do every day - when I come to a stop the idle has a tendency to slowly drop to 500 or less and sometimes will even stall the engine unless I blip the throttle.

If it does stall it starts right back up but it is a bit annoying.

Any thoughts on what this might be? I've checked and the SU fuel pump (new last year) is putting out the correct pressure/volume.

As I said the idle is steady at 850 for the first hour or so.

Could my mixture be a bit too lean or rich? (I don't remember which way could give that result.) Or could it have something to do with that temperature compensator peculiar to the HIF4? I have a feeling this is somehow temp related but I'm clutching at straws.

Thanks! -Greg
Greg Smela

are the poppet valves on the throttle plate soldered?...did you have the throttle shaft bushings changed?...is the vacuum can working on the dizzy?....
pete

Pete -

The throttle plates were replaced with solid and the shaft bushings were changed and are nice and tight.

The vacuum can...hmmm...it was working before but let me check that tonight.
Greg Smela

Hi Greg - I have been battling the same exact symptoms with my 73 for years, and with my 72 before that. I just had my carbs rebuilt, and a new distributor from Advanced distributors but I have yet to install both as my engine just made it back into the engine bay 2 days ago. I am hoping that my issues are solved, but now am less hopeful since you are having the same problems. I will try to let you know how mine turns out. My poppet valves were soldered years ago, but they have been removed during the rebuild. I agree that it must be due to heat soak, possibly affecting the bimetallic mixture compensators. I wonder if those change as the years wear on.

Erick
Erick Vesterback

If the problem is only at idle then it isn't going to be fuel starvation, that would be much more evident at larger throttle openings when the demand is greater.

Mixture going weak *or* rich from the correct setting will cause the idle to drop as the correct setting should give the highest idle.

If its the distributor gradually retarding the timing (advancing it will cause an increase in idle) you should be able to detect that easily enough with a timing light.

My HSs tend to do this when stuck a long time in traffic as they have no temperature compensation, and it is heat soak that causes it. The temperature compensated jets on the HIF are supposed to prevent it and they do on my V8, so possibly one of those is faulty or not seated correctly. If the lifting pin method works when tuning them then again you should be able to detect either or both carbs going rich or weak when you are getting the problem by using the lifting pins.

Paul Hunt 2

Thanks for that explanation Paul. I'll have time to work on it this weekend and I'll let you know what I find.
Greg Smela

Greg-
You said that you have a new heatshield. Are the insulating pads missing?
Steve S.

Steve -

The insulating pads *are* present and unfortunately I couldn't duplicate the problem today.

Perhaps I had some bad gas?

Of course a new problem reared it's head. I had replaced a faulty fuel sender a few days ago and today was the first time I filled the tank since doing so. Looks like I got one of the bad ones.

Of course the darn thing started leaking like crazy where the electrical connector passes through the plate so I'm busy draining the tank...

Oh well!
Greg Smela

Well, I'm probably going to get a lot of flack for saying this, but in 23 years and at least 12 driver-MGBs, let alone all the other MGBs I've driven or worked on, I've never seen an MGB with a stable Idle. They have all dropped in idle speed when driven extensively. Sometimes at a string of 5 or 6 stoplights in slow traffic, the B will idle at a different speed at each intersection. Of course when it gets down around 500, left alone it will die.

I know it's "cheating" but my own solution" to this is to set the idle higher - somewhere between 1000 and 1100. That way, in extended running it generally idles between 800 and 900. Maybe we should set the idle at those higher temperatures when the idle wants to slow down.

Admitted, this is not really a "solution" - just an excuse for not really solving the problem. I have plans to do some performance upgrades on this car, at which time I will have to upgrade the carbs from "OK, considering how old they are" to "fully rebuilt to high standard". I know leaking shaft bushings are much less of an issue with HIFs than with HSs, but I believe that the manual recommends rebushing every 212,000 miles or 34 years, whichever occurs first. I'm also still running the old poppets - should get solid throttle plates.

Greg, ' glad to hear you're still driving and enjoying the car!

FWIW,
Allen
Allen Bachelder

I think you're right Allen. I've always tuned my cars after warming the engine for about 30 minutes maximum.

I think I'll do a complete tuneup on sunday after driving it for a few hours to see if that makes a difference.

I've really been putting a *lot* of miles on the car - 38,000 in two years - so I don't blame her these little peculiarities!
Greg Smela

Allen - yes and no. My 73 roadster with HSs always suffered from a falling idle when stuck in traffic. It only had to fall a bit for it to start shuddering really badly, but if I upped the idle it ran on quite badly when switching off, so I just used to pull the choke a little bit into the fast-idle zone to 'correct' the low idle. When we chganged over to unleaded and lower octane the runon got so bad at even a normal idle I just had to do something about it. An after-market valve was useless - until I plumbed it in like the North American system. Now I can run an idle of about 1000 rpm and get no runon at all, and if the idle does drop when stuck in traffic it only does so to about 800 rpm and so still idles fine.

My V8 with HIFs have never suffered from either a falling idle or runon.
Paul Hunt 2

Talking about HIF carbies I have a set on my 79 "B" and just yesterday I noticed that fuel was leaking from the drain off outlet of the rear carby. I haven;t had a good look at it yet bout could it be a propbem with float height? Love to read comment on the site Thanks in advance Jeff
jeff jorgensen

Greg,

Now you're a SERIOUS driver! About the only downside of retirement is I can no longer boast about daily driving. I don't drive anything daily! No way am I about to rack up 19,000 miles per year. Will try to make up for that a little this year as we'll do 7,000 miles to the West Coast in July, a trip to Minnesota in September, and a trip to Virginia in October.

The interesting thing about Greg's car (for those that don't know, I once owned it) is that the lady who bought it new drove so little, the car only had accumulated just 39,000 miles in 28 years when I bought it. I put about 15,000 miles on it, and now Greg is really making up for lost time. And I'm sure Greg will agree, the more you drive these cars, the better they get. That car had several issues when I bought it - all relating to disuse.

Low-mileage cars do pop up occasionally and the idea seems attractive, but they usually have more problems, not fewer. Unless you're keeping your car as a museum piece (in your parlour, maybe?), low-mileage is not, in itself, of any value - unless it means that the car has been thoughtfully stored in a climate-controlled environment such that the original finish and interior is perfect. Even then, the car will take weeks, if not months of sorting before it gets to the reliability level most of our 100-200K cars.

Paul, thanks for your advice. Of course, high idle = more run-on. I should know that but I've been shutting my engine down with the clutch for years and it never occured to me that the high idle was contributing. Several years ago I bought a replacement oil-pressure switch. I then found out it was the wrong kind, so I've left the anti-run-on valve disconnected. It's been one of those things requiring a "Round Tuit", and I just never did get a round tuit. I'm adding this to my list of things to fix before that 7000-mile drive.

Cheers, all,
Allen
Allen Bachelder

This thread was discussed between 02/05/2007 and 06/05/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.