Just picked up an '85, need carb help

[bobzdar]

Hi guys, first post here - I just picked up an '85 vmax with 69k miles. Description was that it ran well up until last year, then sat over winter and wouldn't run due to the carbs being gummed up. The owner apparently spent a bunch of money on the carbs with no luck (new diaphrams and some other stuff, says he sent them out for around $900 worth of work) and then sent them to someone else who said they needed so much work they weren't worth doing and to just buy some good used ones. He mentioned something about it needing a mixture screw and the jets were messed up? At this point he decided to sell for that and other reasons. Only other issue he said was the somewhat typical slipping in 2nd gear for the early vmaxes.

Now, I have and do all of my own mechanical work and have rebuilt a bunch of carbs but have never messed with Mikuni's, so don't really know their inner workings. I picked the bike up Monday and decided I'd clean up the carbs and put them on to witness for myself what they're doing. I got them on, charged the battery up and somewhat surprisingly it fired up pretty quickly. I had to mess with the choke some to get it to idle but it idles pretty smoothly and with no issues or strange noises. However, as soon as you crack the throttle it dies. Messing with the choke and the throttle I can get it up to about 2500 rpm from the 1k idle before it dies, but it does not appear to be getting any shot of fuel when the throttle is cracked. You can tell it tries to rev up but just dies due to lack of fuel. Now, I left the air box off and all of the vacuum lines off to watch what the carbs were doing, do the vacuum lines need to be hooked up for the carbs to work? Where's the best place to start? My first thought is to check and see if the fuel pump is up to snuff and/or filter is clogged, just pull the line to the carbs and see how much fuel it pumps for a second or two (obviously into a cup)? If the fuel system is ok, where to go from there?

The bike has a supermax seat, progressive front springs, a new clutch and some other stuff, but is otherwise bone stock - dark blue metallic in color. I want to get it running and ride it a bit and then really go through it over the winter.

 

[Buster Hymen]

Put the air box back on for starters as it will have a hard time running without it. Also have the carbs been sycned since you've put them back on?

 

[naughtyG]

Buster beat me to it - airbox must be on or it will die on revving.

Sync is also very important - do you have access to a syncing tool? If not you can make one very cheaply and easily with a board, some clear hose, a few T connectors and a bit of tranny fluid..

Those hoses would connect to the vacuum nipples on the carbs. When not syncing, one of these nipples connects to the vacuum advance hose, and the other 3 must be capped or it won't run at all.

There's been a mod lately we've been doing where we connect all four vacuum ports together and then to the advance unit, and that seems to help both idle and throttle response.

 

[bobzdar]

The vac advance is hooked up and all of the rest of the ports are plugged. Can I just hook a vacuum gauge to the nipples and set them all to the same reading or do I need to use a comparison tool?

I'll put the airbox on when I get home and see what happens.

 

[mr.maxx]

yes u could but it wouldnt be as accurate.

the front carb on the left is the master so set them all to match it.

it will get u by till u get a sync tool prolly.

 

[[email protected]]

Don't forget to make sure the vent hoses are properly routed and secured. It has been known to cause issues.

 

[naughtyG]

The vac advance is hooked up and all of the rest of the ports are plugged. Can I just hook a vacuum gauge to the nipples and set them all to the same reading or do I need to use a comparison tool?

Yes you could use a Vac gauge but it won't be very accurate. That's where something like a Morgan Carbtune (about $100) really helps, but as mentioned b4 u can make your own gauge very cheaply and it works just as well.

 

[bobzdar]

So I put the airbox back on and hooked up the vac lines, couldn't even get it to start. So I pulled the fuel line to the carbs and hit the ignition, fuel pump definitely works. Then I just poured some gas down the carbs and it fired right up and let me rev it once or twice til the fuel was gone, so the issue is definitely lack of fuel in the carbs. I'm not sure if the floats are all stuck shut, or they weren't set right, or possibly the bowls are full but the passages out are blocked. I'll try cracking the drains and see if any fuel drains out which should let me know if the bowls are filling or not. If not, can the floats be set on the bike? It looks kind of tough to remove the bowls without taking the carbs off, not sure which would be more of a pain.

I can't really figure out why it would idle yesterday and won't even do that today, could be there was a tiny bit of fuel making it's way through the carbs? It doesn't really make sense though, the bowls should have been able to fill and then run the bike for a few seconds normally until the bowls drained and then die out and start again once some fuel got into the bowls, but it would just sit and idle and do nothing else. Maybe the floats are just set extremely low.

 

[naughtyG]

TBH, on a bike that age I would pull the carbs off and take 'em apart like I did (check out my photo thread in the carbs sub-forum), inspect every perishable (rubber parts) and renew as needed. Don't use aftermarket carb bits - they don't work well. Use Gary McCoy @ Mondak to get what you need at a good price.
I had all sorts of idling and conking out problems until I did that.

 

[4gasem]

Make sure your tank isn't full of rust as well. The particles get so small they get past the filter and mess up the carbs.

Vmax carbs are a PITA to keep working correctly but it's doable.

One2dmax and Maleko89 both rebuild carbs for a fee and know these things VERY well. That may be the avenue you want to take if you can't get it done yourself.

Chris

 

[bobzdar]

According to the PO, they were already gone through and had new diaphrams installed. I'll start with the bowls and go from there as I'm fairly certain that's where the issue is, but tbh these things are a lot less complicated than the car carbs I'm used to working on so I doubt I'll need to send them out. I was just wondering if it was possible to look at the bowls with the carbs on rather than take them off, separate them etc.

 

[4gasem]

NO! You actually have to split them to get the bowls off. This is why they're a PITA.

Chris

 

[vmax626]

You might want to try some SEA FOAM in the tank with fresh gas and fire it up and let it run.
sea foam is just about the best stuff for cleaning out carbs before gutting them.
It`s about $9.00 at the auto parts and thats what most of us use to clean and maintain clean carbs.
I`m suprised that no one mentioned this yet. Go to the "V_BOOST room and scroll down to the "SEA FOAM" thread.
good luck.
<<Dave>>unk:

 

[4gasem]

His carbs sound like they're WAY past that point IMO.

 

[bobzdar]

I opened the carbs up and they were pretty clean, floats weren't set evenly so I just made sure all of the passages were clean and reset the floats. Then after examination I found that the right 2 carbs are missing the pilot screws, so I can't go any further. Local yamaha dealer wants 20.99 a piece and won't have them for 7 days. Any on-line sources I can get the parts from?

 

[one2dmax]

Lots of online places. I have them on hand but not sure what the current price is (prob close to their price).

Sean Morley

 

[bobzdar]

I sent you a pm, let me know if you can get them to me before thursday of this week.

 

[bobzdar]

Just wanted to revisit this as I put the bike away after getting the pilot screws and not having any luck. I decided to get it out and see if I could get it going this weekend so I could do some riding this spring and am pleased to say I was mostly successful.

The pilot screws weren't installed in 2 of the carbs for a reason, there is a problem with the threads in one of the carbs and the other carb had something preventing the pilot screw from threading in all the way (I think an old washer/gasket is stuck in there). So, I have 3 pilot screws installed and just left the 4th out as I need to figure out how to chase the threads (it looks like just the very first thread is the problem). So, I poured a bit of gas in the carbs and hit the starter just to see if I could get it to idle, and with a little choke applied it would. I then let it warm up completely and to my surprise, it would actually rev up, though not smoothly until 4-5k. I was getting some backfiring and smoke but it would actually run. I re-installed the airbox completely and then did a basic carb sync using a vacuum gauge and played with the 3 pilot screws that were installed and have it running decently smoothly, albeit I have to leave the choke on a little to keep it running even when warm. After playing with all of the settings to the point that it was running it's best, it would pretty much rev properly with only a little bogging and a little bit of backfire. So, at that point I figured I'd better try and ride the thing so I'd know what else I was facing besides getting the carbs fully working. To my surprise, despite a little difficulty off-idle, it ran quite well, though I only took it up to about 5-6k max for now. I'm fairly confident that once I get the other 2 pilot screws figured out, I should be able to get it running properly without the choke on. The more it ran, the better it ran, so I'm just going to keep taking it for short rides until I get everything sorted. Having never ridden a vmax before, I was pretty surprised at how docile it was provided I kept my right hand in check. Definitely handled and felt better than the sportster 1200 I had ridden, which was my closest frame of reference in terms of bike size. I was just happy to ride the thing and make sure everything was working properly.

 

[2fear]

:clapping:Great news man!