5000 rpm idle

Bike running like a dog? This section might well answer your questions. Posting restricted to registered users.
Post Reply
bruce1947
Posts: 4
Joined: Wed Aug 19, 2015 11:16 pm
AntiSpam: No
Location: Quebec Canada

5000 rpm idle

Post by bruce1947 »

This is my first post, this forum seems full of knowledgeable, helpful guys (and girls?). I bought my NS new in Canada in 1986. Much of my working life has been overseas so the bike has more storage time than riding time. I returned this summer after 7 years and found mice had gotten into the air box and filled it with soybean seed (the bike is stored in the barn office). They even got the slides open and seeds were down in the intakes! I took the carbs apart cleaned out the intakes and reassembled. I replaced one of the intake manifolds, as it was cracked, with one that had been covered in a flexible epoxy. The bike started third kick but raced to 5000 rpm then headed to red line as I hit the stop button (fortunately I was using premix and had oiled the cylinders). Fiddling with the cables and adjustment changed nothing. I took the carbs off thinking I had missed something that had jammed the slides, but all moved easily, and down to the bottom when I undid the actuating arm set screw.

I have a few queries: The right carb slide (not adjustable) is open 11/64 ths which is more than i remember, by a lot. Will an air leak explain the high idle and racing? One rubber between the carb and air box is cracked and patched with gaffer tape, are air leaks before the carb a potential source of worry? The vacuum line to the fuel tap has hardened and may leak air. Is there any reason not to block it off at the engine ( I always turn off the fuel tap anyway when stopped). Not long before putting the bike in storage I synched the carbs with a mercury based vacuum guage. At somepoint it tipped over and now the "unobtainium" mercury is gone. Any suggestions for a relacement? The workshop manual I have is great at describing how to disassemble and reassemble but not much on trouble shooting. Other than this forum are there other manuals?

Thanks for your thoughts.

Alastair
Posts: 1784
Joined: Sat May 25, 2002 1:01 am
Location: Glossop U.K.
Contact:

Re: 5000 rpm idle

Post by Alastair »

Difficult one. Air leaks will cause high idling. With lack of use and age I'd expect the crank seals to be stuffed. Try a leakdown test as first point of reference.

I can remould the inlet rubbers if that helps.

Mark
Posts: 1352
Joined: Thu Oct 25, 2001 1:01 am
Location: Up Norf

Re: 5000 rpm idle

Post by Mark »

Alastair wrote:Difficult one. Air leaks will cause high idling. With lack of use and age I'd expect the crank seals to be stuffed. Try a leakdown test as first point of reference.

I can remould the inlet rubbers if that helps.
I agree with Alastair. After standing for so long, there's a good chance that the crank seals could be shot. Air leaks before the carb won't affect Idle, but anything after, such as an inlet rubber, will. It's also possible for for one cylinder to scavenge mixture from the next if the crank seals are shot.

The fuel tap should only flow fuel when the engine is running so best not to disconnect unless you are running the bike from a temporary gravity tank. However, there is a diaphragm behind the plate that the vacuum pipe attaches to, which is held on with four screws. If the diaphragm fails, the fuel tap will probably flow fuel regardless of whether the engine is running or not. I don't think an air leak on the pipe would flow enough air to raise the idle as you describe but it it's flowing fuel down the pipe, combined with another air leak from a manifold rubber, it could. I've never know fuel to be drawn into the engine via the vacuum pipe, but that's not to say that it can't happen.

If this does turn into a major job, fortunately, centre crank seals are now readily available from Ian Wright at Classic Race Replicas and the left hand seal is available from Honda. The only seal which may cause you a headache is the primary drive seal, which Ian may also have.

http://www.classicracereplicas.com

I hope it's a simple fix. Please report back what you find.
If it ain't broke, fix it 'til it is.

bruce1947
Posts: 4
Joined: Wed Aug 19, 2015 11:16 pm
AntiSpam: No
Location: Quebec Canada

Re: 5000 rpm idle

Post by bruce1947 »

Thanks Mark and Alistair.

I bought 3 intakes from a seller in Singapore but only put one on as the the other two seemed intact. I will check them more closely. I will use a gravity tank now but started initially with the regular tank and vacuum line. The engine also raced to 5000 RPM just on fuel in the float bowls (and the tank off) but I forgot to plug off the vacuum line. I will check the vaccum circuit none the less.

I was concerned that the right carb slide, being set at 11/64ths was too high, (not that is adjustable anyway) but as neither of you mentioned this I assume it is within spec.

I will check on the site for how to make bungs for the leak down test but have you any quick thoughts?

Until now the bike has been trouble free; at 24000 km I have had to do nothing other than change the oil and spark plugs, fork seals, chain, brake pads and rear shock - perhaps its time for a major overhaul has come.

I will keep you posted

Mark
Posts: 1352
Joined: Thu Oct 25, 2001 1:01 am
Location: Up Norf

Re: 5000 rpm idle

Post by Mark »

bruce1947 wrote:I was concerned that the right carb slide, being set at 11/64ths was too high, (not that is adjustable anyway) but as neither of you mentioned this I assume it is within spec.
Probably because we've been metric for the last 30 odd years and it didn't register. Around 4.5mm, which is a little high.

You're right, No3 cylinder (RH) has the idle adjusting screw attached to it so when it is adjusted, the throttle slides on cylinders one and two are adjusted by the same amount. I can't remember how high I set mine last time, but I know I used a feeler gauge. Probably between 0.5 & 1mm. What height are the other two set at?

Is it possible there is still some debris causing the slide to stick? But then, as it is the master carb and considering the way the carbs work, you would expect the others sit at the same height. Worth checking as you have the carbs off anyway.
If it ain't broke, fix it 'til it is.

Alastair
Posts: 1784
Joined: Sat May 25, 2002 1:01 am
Location: Glossop U.K.
Contact:

Re: 5000 rpm idle

Post by Alastair »

I agree with Mark. Imperial didn't register with me either!

For bungs, a flat piece of solid plastic sealed with silicone and left to cure overnight will do the job.

Straycat
Posts: 86
Joined: Sat Feb 18, 2012 7:37 pm
AntiSpam: No
Location: Canada

Re: 5000 rpm idle

Post by Straycat »

For bungs, i recently acquired some expandable rubber plugs that are used on pools and hot tubs to seal off plumbing pipes etc. you can get them at plumbing centers, or hot-tub and pool places too.

I've also seen thick pieces of inner-tube rubber used between the manifolds and the cylinders.

for fixed exhaust ports, the expandable rubber plugs work well, they come in a variety of sizes.
1986 Honda NS400r
1985 Yam RZ500
1990 Yam RZ350 YPVS
1987 Suzuki RG250
1988 Suzuki RGV250 VJ21
1989 Yam FZR400
1986 Honda TLR 200 Relfex
1986 Honda VF500F Interceptor
1985 Honda VF500F Rothmans
1985 Hond VF1000R (x4)
2014 Honda VFR800

bruce1947
Posts: 4
Joined: Wed Aug 19, 2015 11:16 pm
AntiSpam: No
Location: Quebec Canada

Re: 5000 rpm idle

Post by bruce1947 »

Thanks for this!

I am travelling now but will be back at it next week. I will keep you posted.

garysaki
Posts: 2
Joined: Thu Jan 21, 2016 8:03 pm
AntiSpam: No

Re: 5000 rpm idle

Post by garysaki »

Hello, this may be too obvious, but are your carbs CLEAN? In order for any vehicle to run well, you must have a clean fuel system, meaning tank to petcock to carbs. Dirty or plugged pilot (slow speed) jets can cause hard starting and then a fast idle. It may run fine from mid to high speed, but plugged pilots won't allow a proper idle.

Gary

Mark
Posts: 1352
Joined: Thu Oct 25, 2001 1:01 am
Location: Up Norf

Re: 5000 rpm idle

Post by Mark »

garysaki wrote:Hello, this may be too obvious, but are your carbs CLEAN? In order for any vehicle to run well, you must have a clean fuel system, meaning tank to petcock to carbs. Dirty or plugged pilot (slow speed) jets can cause hard starting and then a fast idle. It may run fine from mid to high speed, but plugged pilots won't allow a proper idle.

Gary
Correct. Massive over adjustment of one slide could have been to overcome/mask a blocked idle circuit. However, it would be fairly unusual for an NS. Stuffed crank seals on the other hand, are not. As Alastair points out, a leakdown test as a point of reference would be very valuable. It's possible to chase obscure NS running problems for months and spent a fortune doing it, when a quick test could prove seal condition. Just the voice of experience speaking.. :roll: :oops:
If it ain't broke, fix it 'til it is.

bruce1947
Posts: 4
Joined: Wed Aug 19, 2015 11:16 pm
AntiSpam: No
Location: Quebec Canada

Re: 5000 rpm idle

Post by bruce1947 »

I apologize for not keeping you up to speed. I took all the carbs apart again and cleaned, poked, and blew out all the jets and passages.I re-synched them statically but then I was back traveling again and as it is winter in Canada I left the bike in the garage (with mouse traps all around so they will pay a heavy price if they try to get in the carbs again). I will be back home in 6 weeks and will start up the bike.

Post Reply