Triumph TRW running lean

[sexton]

Hi, I'm working on a 1950's Triumph TRW. It's an ex-army bike with 600 miles on it. I think it's running lean and I can't figure out why.

 

Here's what the engine does:

- won't idle, needs lots of throttle to keep running

- backfires occasionally through exhaust

- runs a bit better with start valve on Solex carb selected to richer "warm-up" position

- runs a lot better when carb intake is partially blocked by hand. RPM picks up a lot

 

Here's what we've done so far:

- compression in both cylinders found good at 110 psi.

- stripped down carb. All jets and passages clear. Carb base flange found badly warped, bowed about 1/32". Sanded flat. This improved running a bit, but not as much as I expected. (Still need to check fuel level is OK.)

- hoses from gas tank to carb are clear. Lots of flow when disconnected from carb.

- found timing 24 degrees BTDC at around 2000 rpm, using timing light. Can't check at idle, it won't idle. So maybe a bit advanced? But I don't think that would cause the symptoms I am seeing.

- new plugs. Both checked good on plug tester at 140 psi.

 

It's behaving like it has a bad air leak between the carb throttle plate and the intake valves. But there's nothing there to leak! Has anyone ever seen the intake passages cast into the cylinder pair leak air in through cracks or porosity?

 

Malcolm

[sexton]

Bump. Any suggestions?

 

Malcolm

[Richard Farrant]

Hi Malcolm,

Do you have the air cleaner connected to the carb? If not this will have an effect on the mixture and could give symptoms of it being lean.

 

regards, Richard

[mtskull]

Are you 100% sure that every passage in the carb is completely clear? Even a slight restriction would give the symptoms that you describe. I know it is easier said than done but if you can borrow a carb off another bike and try that, then at least you could either eliminate or confirm the carb as the source of the problem.

[79x100]

You may struggle to obtain much of a response in the motorcycle section. Motorcyclists in general really don't like carbs such as the Solex and most of us would quickly fit an Amal...which doesn't help with a restoration to original of course.

 

These carbs were specified for economy when compared with the typical slide carburetors which often waste fuel if not treated gently so they're probably pretty lean at the best of times. Any air leak on butterfly pivots etc. could cause problems. Have you replaced all the gaskets and seals ?

[Barrie]

Malcolm,

 

Triumph TRW carbs advertised for sale on the MVT site, in the 'For Sale' section.

 

http://www.mvt.org.uk/sales.html

[sexton]

Thanks for the suggestions.

 

As far as I recall, all the runs were done with the air cleaner removed. I've never noticed a difference running an engine with air cleaner off or on, but maybe the TRW has a particularly restrictive air cleaner so I'll try a run with it connected.

 

If that doesn't work, I'm going to pull the carb again and double check all jets and air bleeds. The manual gives some jet and air bleed sizes so I'll check those in case of a substitution or mix-up, although that's unlikely.

 

And thinking about the seals and gaskets as suggested made me realise the unusual Solex starter valve design has the potential to introduce an air leak past the throttle plate if it's not assembled properly or it's warped.

 

And if none of that fixes it, I think the next step has to be check the cylinder casting for an air leak into the cast-in intake passages.

 

Just a brief note on the history of this bike. It was given to the museum I volunteer at as a brand new bike in a crate in the 1980's. It has since accumulated about 600 miles. No doubt some tinkering has been done over the years.

 

Malcolm

[sexton]

Change of plan. After reconfirming the Solex carb is in great shape, float level is correct, all passages are clear, and jets and air bleeds are all to spec, and pressurizing the intake passage in the cylinder casting confirmed it isn't leaking badly, it's becoming obvious my lean-running diagnosis was wrong. So now we're going to look for an ignition problem.

 

Malcolm

[sexton]

After checking out the carb and finding nothing wrong, the young lad working with me took the pieces home and put it back together. When we reinstalled it, it ran much better! Once warmed up, it idled nicely. A bit embarrassing, that.:red:. I think the stacked plates that make up the starter valve on the bottom of the carb may be a bit warped and he sealed them up better.

It still backfired a bit through the exhaust at higher rpm but replacing the failed condenser seems to have fixed that.

 

Next step is to get a battery and take it out for a run.

 

Malcolm