ET: Engine (Twin) Effects of Cam Timing

[greg brillus]

Hey there fellow Vincenteers........have any of you mechanical types any hands on experience with cam timing on the twins where one or both cams were actually out.......that is either set up wrong, or the cam slipped in the gear and caused a running issue. I had a twin here recently with Mk 2 cams, 7.3:1 pistons Shadow 289 carby's...........seemed hard to start, but ran fine in the shop.........out on the road it seemed a little gutless on power and the reversion coming off the front carb, i could feel the fuel hitting my leg........Plus the engine did not even sound normal, almost like one cylinder was down considerably on power.......After an inspection with the timing cover off, it was apparent the front cam was about 1 tooth too far advanced........Set it up one tooth back......now it starts better and sounds more crisp in the exhaust note........have not had the chance to ride it yet on account of the constant rain we have here at the moment, but I'm quietly confident it should go better..........has anyone had similar experience or worse due to the same or similar thing........This could explain why some bikes are hard to start..........any thoughts........Cheers........Greg.

 

[passenger0_0]

Hi Greg. Early on in my journey developing my sidecar engine I put in Robinson 105 cams. Timing wasn't specified and given the re-phashing between the inlet and exhaust lobes had been opened up from factory mk2 cams I couldn't open the inlet valve as early as specified by mk2 timing without having the exhaust valve opening too early. I tried a compromise and found the engine went OK but wouldn't rev above 4,000 rpm on the track. On the dyno, removing the K&N filters revealed significant reversion of the inlet flow occurring with what looked like a fire extinguisher going off with fuel vapour exiting out from both carbs. Replacement of the cams with Robinson mk2 profile and timing eliminated the reversion and engine rev limitation.
Moral of the story (for me anyway) is that late inlet timing causes reversion at higher engine speeds and that advancing the inlet opening timing improves top end power. I know others may have differing views but this had been my experience.

 

[greg brillus]

Thanks for the info David, the other part that is somewhat difficult is the setup on a road bike.......this makes setting cam timing almost too time consuming unless you are prepared to strip a fair bit of the bike to check the timing accurately........Just using some simple checks from one cylinder to another, I could see the front cam was giving way more lift on the inlet valve than the rear cam........For example.......say the rear inlet at TDC was showing about 0125" lift whereas the front cam again at TDC on that cylinder was showing about 0.200" lift.......These figures may be wrong for a Mk 2 cam, but it is just the difference between the 2 that I could see was an issue.........I figured if the front inlet was opening too soon, then the exhaust gases would push the inlet charge back out the carby, and then draw in a charge of diluted fuel/air mix as the piston went past TDC and down the bore........It is my understanding that early race engines like the Manx with big megaphone exhausts suffered from "Megaphonitis" due to this exact problem.......In part a carburettor/inlet manifold tune, verses the tuned exhaust system.......Only the 2 work well once the engine rev's come up, and the exhaust scavenging starts to work........This reversion at the carby bellmouth actually makes the engine loose power because the mixture is so rich from the almost "Double charge" of fuel entering the engine.........It blows the fuel/air out, then quickly draws it back in..........My 665 racer was suffering from this a while back, and I fixed it by lengthening the header pipe before the mega/muffler.

 

[roy the mechanic]

Had something similar a while ago on my norvin. Initially it ran ok but had a fair bit of reversion. Shortly after it refused to run on both cylinders. The long and short of it was the rear cam gear had spun on the cam. Fortunately the valves stopped in the both closed position. Spoke to Terry, was advised to set it up 172 thou at tdc. got it all set, took it apart, welded the cams+ wheels in position, checked to be sure, now starts ok, goes very well,idles like a stocker. Big Sid had similar troubles with his vincati, according to their book.

 

[oexing]

To be open, I do not mess around with funny timing numbers , just set cams for equal lift at 4 up to 0 TDC. I believe the equal lift at near TDC will have most effect on performance, unlike all those funny numbers of valve lift open and closing . At false overlap situations you´ll have all sorts of effects you do not want at all - as reported above. When you look up diagrams you will find that this is correct for all engines, either equal lift at exactly TDC or 4-5 dregrees before. Does not matter what all other numbers in files say.

Vic

some diagrams for touring and racing cams:

 

[Monkeypants]

I'm a 4 degrees off man as well.

Glen

 

[Chris Launders]

On the subject of valve timing being off, a few years ago I sleeved my Brough engine back to it's standard bore of 85.7mm from 90mm, this decreasing its capacity from 1200cc to 1096cc, I had the whole engine apart at the same time.

Since then it had been a bit down on performance but I put this down to the reduction in capacity and the new bores. Anyway a couple of months ago I had the gearbox out to do some work on it and while the primary drive was off I could get a degree disc on so decided I would just check everything, I found the valve timing was 25 degrees retarded (being very coarse teeth on the cams this is only about one tooth).

Fortunately this being a sidevalve nothing can hit or get tangled but it did run surprisingly well for being so far out, of course I retimed it and it is a bit more sprightly but not massively so and has required no adjustment of the carb.

 

[greg brillus]

Yes we are lucky that we get away with many things on these old bikes.........try getting the valve timing out a tooth or two on a modern engine and its all over.........Actually the easy downfall on one of these is to install a Mk 2 cam and think you might get away with it........I'm sure plenty of new cams/followers have been destroyed straight away without checking for travel in the guides first.

 

[chankly bore]

Fortunately this being a sidevalve nothing can hit or get tangled but it did run surprisingly well for being so far out, of course I retimed it and it is a bit more sprightly but not massively so and has required no adjustment of the carb.

One of my all-time favourite quotes. Sorry, it was so long ago I can't attribute it: "A valve timing so conservative as to border on the Fascist."

 

[mercurycrest]

Former VOC PNW Section Member/VincentOwner jack coonrod told me that the Drag Racers advanced their cam timing for lower ETs and retarded the cam timing for higher speeds.