Forum Donations
Search
VTF Google Search
 Click Here

RPM Plateau (Rev Limiting)

P901 Single carb intake, FLAASH Cams, Super Charger, NOS.......
  • Advertisement

Post a reply

RPM Plateau (Rev Limiting)

PostAuthor: Libertate » Sun Aug 25, 2019 1:43 am

I got the Cognito Moto VM34 kit installed on the ‘81 XV750 today. The bike idles like a dream and power seems to be very linear up until around 4,500/5,000 RPM, at which point it sort of staggers and won’t increase in speed or revs. Oddly enough, I can happily rev it to redline in neutral— it’s when I’m in gear (any gear) that this issue presents itself. Initially I thought it could be running lean, but the plugs looked quite dark, so I dropped the main jets from 195s to 175s, but the stumble’s still there.

What do you guys think? Any tips that might point me in the right direction would be greatly appreciated. Thanks in advance!

Libertate
Junior Membership
Junior Membership
 
Posts: 21
Joined: Wed Aug 07, 2019 10:11 am
Location: Hayden, ID Flag
Bike year & model: 1981 and 1982 XV750 Viragos
Sex: Male

Re: RPM Plateau (Rev Limiting)

PostAuthor: nanno » Sun Aug 25, 2019 3:16 am

I suspect, you're getting close to the limits of air-flow with these carbs on the one hand and on the other most likely fuelling sounds rather rich. I run #170 mains with my VM38 roundslides, so you probably would want to go down even further with your mains. 150(-ish) to 160(-ish) regions. Listen for detonation, but all in all these engines aren't likely to burn holes into the pistons. (As I now know, it's more the exhaust valves giving up as they can't keep up with the heat.)
http://viragotechforum.com/viewtopic.php?f=12&t=43227 - Nanno's accumulated findings (and blatant show-off) - infos on my TR1's, my performance and reliability mods and a bit of show-casing of the stainless steel exhausts I build

http://greasygreg.blogspot.co.at (My blog - Greasygreg.blogspot.co.at)
User avatar
nanno
Silver Membership
Silver Membership
 
Posts: 1219
Joined: Thu May 26, 2011 4:06 pm
  Flag
Bike year & model: 1981 Yamaha TR1
1982 Yamaha TR1 Turbo
1977 XS 750 Sidecar
1984 XT500 4Valve (Austrian Tax saving edition)
1979 RD250/R5/DS7-hodgepodge (in bits)
Sex: Male

Re: RPM Plateau (Rev Limiting)

PostAuthor: Ita » Sun Aug 25, 2019 3:41 am

Are the slides lifting wide open? Perhaps your twist grip is not compatible with the Mikunis?

Ita
I'm a black bat baby,
Bangin' on your window pane.
Well I'm a black bat babe,
I need my high octane.

Ita
Junior Membership
Junior Membership
 
Posts: 17
Joined: Fri Jun 14, 2019 11:37 pm
Location: Mid North Coast NSW Flag
Bike year & model: 1996 XV 750 Virago (Café Racer Retirement Project)
Sex: Male

Re: RPM Plateau (Rev Limiting)

PostAuthor: Ita » Sun Aug 25, 2019 4:09 am

I had another thought...

Float levels? or the breather tubes closed or blocked?

Both would cause lack of fuel under load...

Ita
I'm a black bat baby,
Bangin' on your window pane.
Well I'm a black bat babe,
I need my high octane.

Ita
Junior Membership
Junior Membership
 
Posts: 17
Joined: Fri Jun 14, 2019 11:37 pm
Location: Mid North Coast NSW Flag
Bike year & model: 1996 XV 750 Virago (Café Racer Retirement Project)
Sex: Male

Re: RPM Plateau (Rev Limiting)

PostAuthor: Libertate » Mon Aug 26, 2019 5:37 pm

I want to thank you both for chiming in. I didn’t exactly expect the carbs to be “plug and play”, but I was surprised that a kit that’s marketed as being tailored to stock (or very lightly modified) Viragos would have been set up with the richest possible jets. In the end, I ended up going with 165s for the mains and 17.5s for the pilots— the smallest ones provided in the kit. I had limited time, so I wasn’t able to run the bike through various power cycle/spark plug checks, but a cursory ride shows a dramatic improvement. I was able to get the bike up to 80 MPH likety-split and the power curve appears to be satisfactorily linear.

I had no idea a rich condition could cause the symptoms I described. I would have expected a strong gasoline smell or popping on over-run, but there was none of that.

The only issue I appear to have now is with the air mixture screws. I’ve tried running through the dead cylinder procedure of setting them, but I’m really not seeing a significant RPM rise or drop when I enrich or lean out the mixture. I’m fairly confident the pilots are approximately the right size because the bike won’t start without choke but will fire up and idle quite nicely when cold (with very little throttle input) with choke on.

Libertate
Junior Membership
Junior Membership
 
Posts: 21
Joined: Wed Aug 07, 2019 10:11 am
Location: Hayden, ID Flag
Bike year & model: 1981 and 1982 XV750 Viragos
Sex: Male



Post a reply


  • Advertisement

Return to Virago Help - Single Carb / Power Adder

Who is online

Registered users: No registered users