Engine vibration at high RPM

Post Reply
Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Engine vibration at high RPM

Post by Zed » Thu May 02, 2019 11:51 pm

I have been mucking around with the carbs to set the mixture and the idle. I used Macl's trick of riding around with the fairings off and a screwdriver in the pocket, taking it down a country road and trashing seven bells out of it. This all cam about due to an observed inability to accelerate for the overtake on the A169 to Whitby on the bank holiday weekend. Reading some of the comments here showed it to be a leanness issue, that I had to correct, but after a days worth of messing it would not idle evenly, putting in neutral would have it spinning at 2000rpm, then dropping to 500 after a few seconds (something I had never seen before). Solution was to shut off one rotor with the idle adjuster and get the mixture adjusted one carb at a time, then balance them by ear. Initially this did not appear to work but after some trashing she was idling evenly at 500rpm. I richened the mixture while riding by 1 quarter turn, which really brought in the acceleration, but by adding 1 more quarter turn acceleration was better in the mid range above 4000rpm, but worse at the top above 7000rpm. Is this usual or is there something wrong with the carbs?

Also I was noticing a high frequency vibration, not very strong but noticeable, as I approached 9000rpm in all gears. I don't think it was doing this before, but I have never really gone fishing at the high end (don't normally ride it like a race bike!). Is this an indication of something I should worry about? I am using Shell V-power, so it should not be a pre-ignition issue...and the engine is perfectly smooth up until the point.


Any thoughts chaps?

BlackIP2
moderator
Posts: 61
Joined: Thu Jan 12, 2017 1:32 am

Re: Engine vibration at high RPM

Post by BlackIP2 » Fri May 03, 2019 11:21 am

I've got the IP2 which is air cooled but has the same carbs as your Commander and also had poor running problems.
Regarding the mixture, I'm pretty sure you should be doing this by taking the carb tops off and setting the jet height to an exact measurement as explained in the workshop manual. As I understand it, SU carbs on rotaries do not operate as they do on 'normal' engines and the mixture screw cannot be reliably set by screwing it in or out to get a smooth idle with the engine running.
Apparently this used to be a problem in the police workshops where mechanics, who were experienced on tuning the SU carbs on police Rover and Triumph cars, couldn't get the rotary bikes to run properly.
However, I might be wrong and water-cooled Commanders might be different?
Can't help on your high revs vibration. Don't think I've ever taken mine to 9,000!

johnbirchjar
moderator
Posts: 1196
Joined: Sat Mar 07, 2009 11:20 pm

Re: Engine vibration at high RPM

Post by johnbirchjar » Fri May 03, 2019 11:23 pm

Reving to nine grand??!!bloody hell,I'v never reved the old Classic that high, even on track days!!Ibet your old bike is realy getting a shift on at those revs.(to cure the "tingling" don't take it to nine gand wink)J.B.

johnbirchjar
moderator
Posts: 1196
Joined: Sat Mar 07, 2009 11:20 pm

Re: Engine vibration at high RPM

Post by johnbirchjar » Fri May 03, 2019 11:41 pm

and in addition to the previous entry.My Classic idled like a sewing machine at 800rpm,unlike the p***y Commander,which "hunts" on idle,(and all the way up to about 1500rpm) and is an absolute pig in traffic evil,(very much like the F1 I once rode,once was enough,thank you.J.B.

Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sat May 04, 2019 9:53 am

Mine has one of Richard N's electronic ignition systems and it is rock steady at idle now, especially when hot. I was told when beding the engine in to run it through the whole rev range. Also my limited understanding of cabs is that the throttle has to go all the way to ensure all of the jets are brought in. The only way to check this is correct is the open the throttle under different loads and RPMs. At 9000rpm it will take you to 70mph in 2nd gear on a country road and it is great for practising trail braking into corners (pretty much essential given the poor front brakes). This not a usual occurrence for me; i usually do not ride with more than 3/4 throttle max or over 5000rpm. 4000rpm is where the fuel dip is hence the most efficient place to ride, but as i said in my first email on the A169 i was having to drop 2 gears and spin to over 6000rpm to any form of acceleration out f the bike. The problem with lots of low rpm riding is the carbon build up and the recommendation from Graham had been to push the the rev range periodically. I have checked the spark plugs and they are a nice dry chocolate brown. I have re-gapped to 0.7mm and see how it goes.

And yes, the big old girl is seriously shifting at that speed and all of my sports bike skills are required to keep her in that thin sliver of tarmac. Sounds glorious though.

Zed

Clive603
moderator
Posts: 8
Joined: Mon Oct 15, 2018 9:42 pm

Re: Engine vibration at high RPM

Post by Clive603 » Sun May 05, 2019 12:39 am

Messing around with carb mixture settings by ear is never a good idea with multiple constant depression carburettors. You end up chasing one error with another so its pretty much impossible to hit a reliable setting that works over a range of conditions. Finding todays sweet spot is possible but good chance it won't be tomorrows. Especially on a rotary which has so little internal friction that it barely loads the carburettors enough at idle to get any sort of proper setting control anyway.

CD carbs as a breed and SU's in particular are very good at self regulation given half a chance so correct technique is to set the mixture by external reference, preferably a dial gauge, then pull the idle in. The bimetallic temperature compensation device adds another layer of self balancing complexity. Its quite easy to get a situation where things will work OK over a limited temperature range then progressively drift out. It gets hot under those Commander body panels. Not looking forward to pulling mine back to "dead right" from "OK I'spose" after the obligatory Italian tune-ups.

My usual riding style was similar to yours back in the days when I was commuting serious miles and I never had carb tuning issues. They just stayed put.

Best thing to do with the brakes if you actually want to stop is to fit 4 pot FJ 1100 / 1200 ones. Also found on other Yamaha models of similar vintage I believe. Both mine got that mod very quickly. Only needs a very simple bracket instead of the direct mount. I have drawings somewhere. Maybe even a spare set of brackets as I made half a dozen sets more years ago than I'm gonna admit to. I like anything I ride to shift sharpish when asked but its imperative that they stop just as sharpish too. I had my FJ calipers hard anodised when rebuilding before fitting. Easing things out so the piston clearance wasn't a touch tight took patience but at least the lethal corrosion under the seal "feature" was gone.

Clive

Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sun May 05, 2019 10:46 am

I see more than one person has modded the front brake with Brembo calipers. They appear to be attached by a bracket as you describe; have not managed to get any information on it and one rebuilt out in Canada had floating discs to boot!

Having asked a few people how to do this and who know how to do this and have the right equipment, i have been told that tuning these carbs is not like tuning SU carbs on a car. I cannot find anyone for at least a month. My view now though is that as long as it is on the rich side it should not damage the engine and i can keep away from areas of operation that i worry about. I had managed to tune it all the way to give me a little over 47mpg; cannot really get any more at i am tall, have a tall screen and have large panniers but in terms of ride-ability past the cat & fiddle, its much better now. Overtakes are now not so scary, you just have to be prepared to reach for another gear mid-overtake, which is either a two fingered (hydraulic clutch) affair.

What is the bolt spacing for your bracket? I will start looking for FJ calipers. Always good to have options as i have spent the last two weeks doing emergency stops so that i am familiar with the brake profile and stopping distance (which is much further than any car infront that is determined to stop)

Clive603
moderator
Posts: 8
Joined: Mon Oct 15, 2018 9:42 pm

Re: Engine vibration at high RPM

Post by Clive603 » Sun May 05, 2019 12:00 pm

When mounting the FJ calipers I elected to retain the standard bottom mounting bolt position and swing the top back to get the pads contacting the right part of the disk. The FJ calipers are longer than the standard ones so the top mounting needs to be higher than the original fork leg boss as well as further back. According to the drawings I've just unearthed its 0.63" higher and 0.40" further back. My bracket design fits on the two original mounting holes with a suitably positioned and proportioned "ear" above the top bolt to carry the top mount. It looks very neat.

I also thinned the mounting bosses on the caliper down by 0.069" to get the disk dead central. Which may well have been over refinement but if going to that much trouble might as well get things right. I used PFM cast iron disks which are a little thicker than standard so clearance with new pads was quite limited. When a bike goes out pretty much every day rusty cast iron disks aren't an issue.

My drawings show three versions. Type one from solid aluminium alloy 0.85" thick with 0.566" thou deep sectors milled out to take the standard fork bosses. Type 2 being either steel one, which I used myself, made from 5 mm plate with the spacer bobbin for the top bolt welded on or stainless steel with the spacer bobbin held on with loctite. The third version, also steel or stainless, has a filler tube in the top fork boss so that an M8 fine bolt, fitted from the wheel side of the fork, and dome headed nut can be used instead of the M10 fine bolt needed to fill the standard hole. An M10 bolt in that position needs its head thinned down and reduced to 14 mm across the flats for clearance. Modifying bolt heads is no issue for me as I have the requisite gear to easily cut hexagons on my Bridgeport but not all folk have access to such.

Type 1 is dimensioned in imperial, types 2 and 3 are metric. Not a clue why. there are some subtle differences between type 1 and the others too. Probably to avoid silly decimal places.

Also included in the drawing set are dimensions for Goodridge hoses, a mount for a proper M10 threaded three way splitter (instead of those appalling never to be sufficiently cursed banjo thingies), a stainless steel bracket and rubber bobbins to stop the hoses attacking the mudguard.

Drawings are dated May and September 1996. Probably got the original CAD files somewhere but probably can no longer read them (MacDraw). If you want I can scan and send but of course everything is trapped on hard copy.

Clive

Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sun May 05, 2019 12:15 pm

Any additional information would be gratefully received. Would you mind posting a picture of your front wheel setup so I can put sopme of what you are describing in context?

Zed

Clive603
moderator
Posts: 8
Joined: Mon Oct 15, 2018 9:42 pm

Re: Engine vibration at high RPM

Post by Clive603 » Sun May 05, 2019 1:08 pm

Didn't know we could attach pictures. How is it done?

Probably easier to PM me with your E-Mail and I'll send the only useful one I have direct. (Front of my first Commander looking every inch of 80,000 miles scruffy in late winter.)

Actually as mine wears MZ 1000S forks and wheels as part of Richards Supreme Commander updates I should really consider selling the front end and all the old parts as its never going back to standard.

Clive

Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sun May 05, 2019 4:53 pm


Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sun May 05, 2019 9:59 pm

Image

Zed
moderator
Posts: 29
Joined: Wed Jun 07, 2017 7:50 pm

Re: Engine vibration at high RPM

Post by Zed » Sun May 05, 2019 10:06 pm

So you can embed images on this forum. This is the picture of the Commander with the original fork but custom front brake and rotor. Also the 1010 prototype that recently sold had the same Brembo caliper:
linkhttps://www.classicsuperbikes.co.uk/cat ... type.html#

Post Reply