Hi
I have a comp 350 which belonged to my dad and has been in the family for 40 years. I used to ride it round the lawn when I was a teenager 35 years ago and spent many a happy hour back then learning how to mechanic and use spanners on the comp rather badly. Anyhow thankfully I am a lot better engineer these days and have finally got it all back together and I gave it its first proper run out last night, a good 20 miles and I it was great.
Pretty trouble free barring the detonation which at first I put down to unleaded fuel although I have added an octane booster. I timed it using a degree wheel at 36 degrees advance, but on the road at part throttle and full advance you could hear the engine continually pinking, retarding the ignition by about 5mm on the lever alleviated this but the engine was not quite so sweet and didn't pull very strongly. Under full load the engine would take full advance.
Well I have been puzzling about this and on inspection of the bike this evening. I noticed that head gaskets were only very thin copper, probably less than 0.3mm thick, now I can only assume that these were fitted by me as a youth from a parts box for another old Douglas my father had a plus 90 racer, which is my next project. I understood about annealing even back then and probably didn't have access to anything else and am wondering if I fitted these or maybe I just thought it would make the bike more powerful.
Now I am thinking that this is actually my problem as the compression ratio is probably too high for the engine causing the detonation on full advance. So my question is what is the correct head gasket thickness for the comp? and should they be copper or the other gray non metalic material?
Thanks Andrew
PS Will post some pictures of the comp soon as it's one of those that's never been seen.
Hi Andrew,
The engine in the Comp should be exactly the same as a Mark engine except for the compression ratio. This was lowered by using longer barrels (50 thou longer than standard). I'm using standard Mark barrels on my Comp with 18gauge aluminium compression plates and paper gaskets instead of the original brass shims. The normal head gaskets would have been the grey composition material (Klingerite) - again about 50 thou thick. With the lowered compression ratio, the engine should run sweetly on full advance on the open road - some retarding will, however, be beneficial when 'trickling' through a 'section'.
With regard to the fuel - when we were forced to use the 'dreaded unleaded' the prophets of doom led us to believe that the valves and seats would be rapidly eroded and that we should use 'octane boosters'. I found that with Castrol Valvemaster added , the fuel left in the tank soon went stale, so for years I have run on plain unleaded with no noticeable ill effects. I have, however, found that the bikes run better with a bit of carb adjustment - a slightly smaller slide cutaway and an increase of 5 on the mainjet - then compensate by lowering the needle a notch or two. This results in a cleaner pickup from tickover and a cooler running engine on full throttle.
Regards,
Eddie.
Hi Eddie
Thank you very much for that information. I will change the head gaskets and see if that cures the detonation.
Interesting about the carb mods I didn't think the jetting seemed quite right, even though the carbs are balanced it doesn't pick
up very well from tickover. I have already tried various settings on the idle and was going to have a look at the needles next.
Also the slides are quite worn and I was considering replacing them. So I might just have a look at your suggestion.
Many thanks Andrew
The slide cutaway will affect the pick up from the idle position. The needles are next & then the main jet at full throttle. There are percentages for each in relation to the total range, but I can't recall them. These are general rules, so heed the advice of marque experts. I also suspect that over advanced ignition could be contributing to rough running; providing other mechanical settings are OK. HTH.
Hi Hampshire Biker
Yes thanks for that but I am pretty certain I got the ignition timing spot on. I spent a lot of time doing it. I stripped the heads off this
evening and sure enough the head gaskets were about 0.3mm copper which I think is way too thin. Also taken the opportunity to grind the valves in and give it a decoke. Hoping to have it back together tomorrow with the right head gaskets and if this cures the problem then I will certainly be having a look at the carburation.
All the best Andrew
Hi Andrew,
don´t worry about the thickness of your head gasket; it has a minimal effect to the compression ratio. Also the unleaded petrol is no problem, because when the T35 engine was produced, petrol was NOT added with unleaded components.
To decoke and looking to the valves will be good. But the main think you should look to your carb settings, and wear (as Hampshire biker has written).
The "modern" petrol seems to be a bit "thicker", so the standard setting could be too weak, specially when your slides are worn.
Regards Michael
Hi Michael
You could well be right, the compression on the comp is very low anyway to start with. Well I did change the head gaskets, decoked the heads and lapped the valves in and having given it a run again yesterday had more issues. I noticed one of the floats was not shutting off properly and on further examination found an quite a lot of debris in the bottom of the float bowls, I imagine some of it has probably been sucked up into the idle circuits and carb bodies.
I thought the petrol tap filter looked iffy to start with but I had fitted a couple cheap square inline filters into the feed pipes, these just werent stopping any rust or dirt at all. So back to the drawing board I have ordered a new fuel tap/filter, a couple of new inline fuel filters, washed the tank out and am going to give the carbs a full strip down and clean out. So we will see how it is then.
Who said 1950's motorcycling was easy.
Thanks Andrew