21 replies to this topic

[alpder]

Trying to think of anything else that could make the idle erratic other than fuelling/air leaks - one thing that occured was something loose/worn/broken in the distributor allowing the timing to wander at idle.  What distributor are you using and where did it come from?

 

Distributor is the original Metro electronic with vac. 65DM4. With a basic timing light it shows steady as a rock at all rpms, and advances smoothly. Even so, I might send it off to Distributor Doctor for a check-over, just in case, if nothing else solves these problems - especially the sudden cutting-out.

[alpder]

If the float is set too low like the picture the fuel level will also be low and it won't run as it's designed to do. 

 

With all the issues present in this carb there's plenty of things that will cause poor running

I've reset the float level and still I can't get it to idle nicely. The remaining carb issues (bad seals on the spindle, needle now probably over-rich at cruise) don't seem enough to make it impossible to get a good idle setting. But maybe spindle air-leaks matter more than I think they do. There is, after all, a "good" clearance at the bushes: 0.2mm slop in all directions which means 0.1mm gap all around the spindle, both sides. And those spindle-seals were doing nothing because they were fitted back-to-front.

 

Today's job is whip the manifold off and check it's airtight. But I'll be amazed if it isn't.

 

Edit: I've also tried blocking the CCV pipe, makes no difference to the idle. But just in case, there's been a new oil-filler cap on the engine and a new O-ring on the dip-stick since a thousand miles ago, too. You can feel the suction as the CCV pulls the filler-cap onto the engine. So leaks in that department aren't it, either.

Edited by alpder, 23 March 2025 - 09:52 AM.

[alpder]

So... I've fitted several new parts, most important one being the correct fuel-seal on the starter/choke. Amazing what a 50p nitrile ring can do...

 

Set the idle speed and mixture by ear (the mix screw has an observable effect on the RPMs, now the engine isn't drowning in petrol at idle). Brimmed it, gone for 100-mile run around the beautiful winding A-roads in our hilly county, and brimmed it again at the same pump.

 

MPG over the last 3800 miles: 29.1.

MPG over today's 100-mile test-run: 38.7.

 

Which is a 1/3rd improvement. That's like having an extra two gallons of range in every 6-gallon tankful. For free.

 

Almost all of that saving comes, I suspect, from low-throttle and - especially - downhill over-run conditions: the carb is no longer dumping the fuel into the engine when manifold pressures are low like it's going out of fashion. What all that raw fuel wash has been doing to my bores and rings I hate to think.

 

Thanks to everyone, as always, for the advice.

Edited by alpder, 26 March 2025 - 06:05 PM.

[bpirie1000]

What do the plugs look like? Compression test now may be a good idea if you do have concerns about rings and pistons. Gives you knowledge that you can compare with later on down the line..

[cal844]

What do the plugs look like? Compression test now may be a good idea if you do have concerns about rings and pistons. Gives you knowledge that you can compare with later on down the line..

I agree with you on this, especially with it being a fresh rebuild and unknown fuel settings

[alpder]

 

What do the plugs look like? Compression test now may be a good idea if you do have concerns about rings and pistons. Gives you knowledge that you can compare with later on down the line..

I agree with you on this, especially with it being a fresh rebuild and unknown fuel settings

 

Plugs are darker than they should be but probably needs a lot more miles to get rid of a year's excess carbon.

 

A quick calc (based on mileage and excess fuel consumption) says that I've filled with 130 gallons of fuel so far but I should have used only about 100.

 

If - as suspected - most of this went through the engine unburnt on the over-run, that's 30 gallons of "degreaser" applied to my bores and rings. (Costing an excess of about £185... another "benefit" of this bad carb.) On the glass-half-full side of the equation, this washing has probably been intermittent and in no-load conditions.

 

I agree there's probably some excess wear. But engine runs sweet as a nut now and I'm not going to stress about something I can't go back and change. Good idea to log the compressions though, for future comparison: with a cheapie gauge I get 208, 215, 207, 216. Which seems at least consistent.

Edited by alpder, Yesterday, 07:50 PM.

[timmy850]

Just buy some new plugs, then keep an eye on them. They’re so cheap and easy to replace