Hi John,
"trouble free motoring" ?? Are you sure this is a Lotus you're talking about ?
Ok, joking aside let me start by saying I know very little about Strombergs and the last car I had with them was well over 20yrs ago so you'll know to take my comments as "guesses" rather than super accurate knowledge.
First the plugs - the N9 is a hotter plug than the N7 series and the N7 is better for an engine where you're revving harder. I'd expect the N7 to soot up quicker than the N9 if you do a lot of commuting, low speed stuff, etc. Referring to Miles Wilkins' book, he says the N9 was standard with your engine & the N7 for "sustained high speeds". However he later adds that N7's can also be used if you have electronic ignition modules fitted which give a higher voltage/hotter spark.
Next, John's suggestion of checking the idle screw position and slack in the cable are the first port of call, it may be as simple as that !
If that doesn't work one thing you can do without any dismantling is to check the timing at idle. There's a lot of detail in Miles Wilkins book but in case you don't have it, here is the data for the Federal Stromberg, 41225A distributor with the vacuum retard fitted;
Below 2000rpm, 5deg, static timing & no advance
2500rpm : 7.5
3000rpm : 9.5
3500rpm : 12.0
4000rpm : 14.3
4500rpm : 16.5
5000rpm : 19.0 max advance
Some of those numbers look academic to me, I couldn't tell that sort of accuracy from a timing light on the engine ! But it should give you an idea where you are.
If the previous owner had removed the vac. retard and/or fitted the UK 40953 distributor, the numbers will be a lot different. NB - this shouldn't be on your car as exported, but Lotus owners are known to fiddle with stuff......
Below 1000rpm, 9deg, static timing & no advance
1000rpm : 14.6
1500rpm : 19.5
2000rpm : 25.0
2500rpm : 25.8
3000rpm : 26.8
3500rpm : 27.6
4000rpm : 28.5
4500rpm : 29.5
5000rpm : 30.5
5500rpm : 31.5
6000rpm : 32.0
6500rpm : 33.0 max advance
Final comment from Miles' book; he explains the development of the Federal Strombergs and the differences from the UK versions. The advance curves were modified to give a quicker warm up and pass the emission tests. The downside was that 19 deg isn't enough to get the best from the engine and he does mention having seen examples where the static timing has been raised to 14-15 deg to give a max 29 deg advance on the 41225A and get it closer to the UK spec engine.
In conclusion, assuming all parts are working properly (and they should be, that engine looks amazing) then my checks would be in order of least work
Ignition is easy - check what you've got compared with those ranges above - timing light and running engine is all you need. Have a look at the distributor and see if the vac retard has been removed or the distributor changed for the UK spec, you'll need a mirror (old school) or ideally one of those USB fibre optic leads going to a laptop which makes it easy to read numbers.
This should give you an idea if the timing is right/wrong/modified. Also if it doesn't change with revs, the advance could be sticking as you mention.
Next would be carbs and if we've still got a high idle after checking the adjuster, then move to see if the linkage is allowing both carbs to close properly.
After that we're into more work.
I'm very hazy on the mechanics of this with your carbs but my guess would be to remove the air filter box and check the air flows through both. which would show if they are balanced properly on the idle adjustment and linkage. The flowmeter like John suggests is ideal for this. As an outside chance maybe look for air leaks from the manifolds, air box, etc.
Valve clearances timing would be my last check, I'd do the cam timings first but I'd concentrate on ignition/fuel then cams.
Finally, I'm going to spend your money (I'm good at spending other folks. money
). Firstly I'd recommend Miles Wilkins book on the TC engine. It's mostly about rebuilds but has loads of technical details on all the variants and also explains where the official period Manual has errors. Well worth the money.
If you have to tinker with carbs then a device to measure air flow is very helpful. Colortunes are good (I have them) but air flow shows how the individual carbs are passing fuel/air into the engine and makes them very easy to balance. I have the one in John's link and it's worth it's weight in gold !
Brian
