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MG MGB Technical - SU HIF4 mixture - winter v. summer

My 74B with new HIF4 carbs run noticeably lean when air temp. is less than about 45 degress F. I thought HIF4's having the temperature compensation device on the jets will not show this tendency. Question is - does the jet temp. compensation affect only mixture changes at high and very high temperature ranges or is it also supposed to remedy the condition described? Any info/opinions appreciated.
willie

I complete the question with: how to be sure the compensation does work correctly?
BC Bernard

Hi Willie.

How do you know it is running lean ?.

You might like to investigate the issue of dashpot oil, a slightly lighter oil might help.

Don
Don

Dashpot oil level is fine. How I can tell it's lean? After a highway run at 3000+ rpms, disengaging the clutch causes the engine to almost cut out - to very low idle speed - 500 rpm or less. In the summer, with same mixture settings and throttle plate position, disengaing the clutch after high engine speed will bring the revs back to the normal 850 rpm. Thanks, Willie
Neil Courtney

Hi again.

Well, cold air is more dense than hot air, so driving in cold weather will tend to result in a leaner mix.

Engine oil is obviously thicker at low temps, but I wouldn't have thought that was a factor after a run.

Having thought about it I doubt if dashpot oil grade would make any difference at tickover.

What are your typical winter and summer air temperatures, do you have a wide range where you are ?.

Don
Don

I stand to be corrected here but my impression was that the compensator leans things out when things get too warm under the bonnet, rather than richening them when things are cold. That may sound like the same thing but in my head it's different, please believe me! What I'm saying is that it's not a surrogate auto-choke.

By way of example, with HIFs on a hot day, stuck in traffic, I see the idle rise slightly whereas with HS4s, the engine would need tickling with the throttle to keep it alive. Starting from cold, there is no difference between them and they both need plenty of choke.

What thermostat are you using? Sounds like you may be getting a little too much cooling at speed. Otherwise, just up the idle a touch for the winter.
Steve Postins

Sorry Neil Courtney, don't know how my last post got signed on under your name. Thanks for the info everyone. I do keep my idle speed on the low side - about 800-850 rpm. Thermostat is 180F. During my driving period, air temps. in NYC range from 90+ F to around 30F. I'll try increasing idle to around 1000 rpm and see if the return to idle improves. But it's snowing here right now, so that'll have to wait.
willie

Willie, keep in mind that the viscosity of the dashpot oil changes with outdoor temp. That change will affect how rich or lean your carbs will run at any given outdoor temp, probably even after a good run. All things considered, it would be worth trying a good synthetic like Mobile1 just to see what happens. Its a whole lot cheaper than changing out the temp compensating devices and may help enough to call it good. If its $7 wasted, then let me know!


Jeff Schlemmer

I've used a V8 with HIFs on a daily basis in very hot and pretty cold temps and found no need to tweak them winter and summer, however the roadster with HSs does benefit from half a flat or so of richening in winter, I have found. Also like Steve my roadster tends to lose a little idle speed when stuck in traffic in summer, whereas the V8 never has. This is opposite to the sympton Willie has.

The HIF temperature compensator may well raise the jet to cope with high summer temps, or maybe just warmer fuel in the float bowl, but by the same token surely that means it will lower the jet and hence richen the mixture in colder temps?

The HIF mixture adjustment screw acts directly on one end of the temperature compensator as I recall, with the other end adjusting the jet, so if you can see the jet move when turning the screw it should be OK. Unless you have copy jets with solid metal in place of the bi-metallic ajuster, of course. Not that I've come across these, but anything is possible. In that case only removing and heating/cooling them and measuring the change in angle of dangle will reveal the truth.

Can't see how dashpot oil would cause Willie's problem as it only comes into play when pushing on the throttle to accelerate. When lifting off the damper is disabled and the piston should drop instantly. Maybe the piston is binding on the cover and sticking up a little, which will cause a tendency to die.
Paul Hunt 2

"surely that means it will lower the jet and hence richen the mixture in colder temps?" What I had wanted to say before is that it's going to depend on the temperature range it works over. My suspicion is that it only comes into play at the warmer end of the scale and is there to stop that hot idle stumble on the HS4s.
Steve Postins

Unless it has a physical stop one end or the other, and I can't recall one, it will bend both ways within limits, which I'd expect to be outside anything we are likely to encounter. It would be interesting to find out at what temperature it is flat, it will then move either side of that.
Paul Hunt 2

OK, my HIFs are off the car and I couldn't resist. I've measured the change in jet height against temperature. As Paul suggested, the rate of change was linear between 10*C and 40*C, with the jet moving by 6.5 thou for every 10*C temperature change.


Steve Postins

Thanks everyone for your efforts. Looks like I need to change to a lighter damper oil from 20W50. If there is a chance to drive on the highway sometime soon, I'll post the results.
willie

Willie, changing the damper oil will do absolutely nothing to affect idle speed as pointed out by Paul Hunt. All this does is control the rate of rise of the dashpot piston and so provide enrichment for accelleration. All you can do is to ensure that your mixture is set spot on after say a 20 mile run and then live with it. Your only solution will then be to perhaps put your idle speed up slightly.

By the way have you checked your throttle spindles and bushes for excessive wear as this can cause unpredictable and varying idle speeds. You will be surprised at how stable an idle can be achieved if they are renewed.
Iain MacKintosh

Hi Steve,

That would mean that best medium adjustement could be obtained by 20°C to achieve a plus or minus enrichment correction from the temp. correcting device upon temp. variations? And what procedure to be used to check out the correctness of this device
Renou

Hi Renou,
10-40*C was only the range I was able to test over - I can't say where the mediun point will be as it was still moving at either end of the range. I tested by putting a dial test indicator on top of the jet whilst the carb warmed on a radiator and double-checked by re-measuring as it cooled. A thermal probe fitted neatly in the overflow pipe and gave the temperatures. The jet rises far enough over this range that you would see the change with the naked eye if you needed a quick check.
Steve Postins

Many thanks. As usual every answer was really relevant.
I need some help. We recently discussed about a Datsun five gears mod on a B, by last October I believe . I can't get the thread back. Could some one help me please ?
Renou

This thread was discussed between 04/12/2005 and 15/12/2005

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