'88 V-Max starts only with choke, revs high for 1 second, then dies.

[Paul S Wilcox]

Hi folks --

The short version of my problem is in the headline:

“88 VMAX starts only with choke, revs high for 1 second, then dies.”

I bought a 1988 VMX12 which had been sitting for years. On the day I looked at the bike, it fired right up using starter fluid. So I bought it, knowing it needed a complete going over.


Here’s what I did:


-- All the obvious stuff: New battery, fresh fluids and filters

-- Cleaned the tank and replaced fuel filter and lines

-- Tested fuel pump

-- Rebuilt the carbs, per the service manual. Spotless!

-- Bench-set the butterfly valves

-- Confirmed proper choke operation

-- Set floats – dry-set on the bench, confirmed with the wet method on the bike

-- Set the air screws at 2 ½ turns

-- Synched the carbs


The problem...


--- The engine STARTS with the choke on, REVS to about 3,000 rpm, then DIES 1 second later


Has ANYONE had this exact problem and found a solution? I’ve been combing the forums for days on end!


Ride safe,


Paul

 

[Fire-medic]

Sounds like you need another cleaning, like it wasn't thorough-enough.

The carbs cannot be properly-synchronized without the engine running. You can set them up so they should allow the bike to run, but then a running bike needs to have the synch done. Check all your boots from the VBoost and the airbox are properly-seated, and tight, no vacuum leaks.

Could you see through your pilot jets?

 

[85vmax]

Hi folks --

The short version of my problem is in the headline:

“88 VMAX starts only with choke, revs high for 1 second, then dies.”

I bought a 1988 VMX12 which had been sitting for years. On the day I looked at the bike, it fired right up using starter fluid. So I bought it, knowing it needed a complete going over.


Here’s what I did:


-- All the obvious stuff: New battery, fresh fluids and filters

-- Cleaned the tank and replaced fuel filter and lines

-- Tested fuel pump

-- Rebuilt the carbs, per the service manual. Spotless!

-- Bench-set the butterfly valves

-- Confirmed proper choke operation

-- Set floats – dry-set on the bench, confirmed with the wet method on the bike

-- Set the air screws at 2 ½ turns

-- Synched the carbs


The problem...


--- The engine STARTS with the choke on, REVS to about 3,000 rpm, then DIES 1 second later


Has ANYONE had this exact problem and found a solution? I’ve been combing the forums for days on end!


Ride safe,


Paul

Is the airbox on.?

 

[dannymax]

Very difficult to clean the pfj's properly. Just for elimination purposes try some new ones from Sean
[email protected]

 

[Paul S Wilcox]

Thanks, Fire-medic, 85VMAX and dannymax.

When I rebuild the carbs, I strived to keep my workspace cleaner than a surgical suite, I read and re-read the manual and posts, replaced lots of parts with new, etc.

As for boots, they are all new, soft and pliable. And yes, I could see light through all of the jets during the rebuild.

I forgot to mention that when I first started the bike after the carb work, it idled fine, provided the choke was on. That's when I synced the carbs and got 'em all pretty close.

Before I dive back into another carb rebuild, I'm checking the things I can from outside the carbs. So far, I checked the Pilot Air Jet #2 (the one under the diaphragm), the Pilot Air Jet #1 (the one in the "throat" of the carb intake), and blew compressed air through the channel that links the two. I also pulled the air/fuel screws, blew out the passageway, then put the screws with spring, washer and O-ring back in and set them to 3 turns out. Finally, I replaced the slide needles, just because I discovered a new set lying around. And of course, checked the itty bitty O-rings under the CV caps. (By the way, I did put the airbox on, but without the clamps.)

Result? ARRRRGH!!! Now it "idles" at 4,000 RPM even with the idle screw backed all the way off. It also pops and coughs, but it does tolerate a little throttle blipping without dying. It also keeps running (sort of) with half-choke.

Here's the kicker: You were right about the possibility of air leaks -- I sprayed starting fluid on all four carb boots and the revs went up like crazy. I was SO confident that I installed and tightened the new boots correctly. Wow was I wrong!

Any suggestions before I jump into carb rebuild #2?

Thanks much,

Paul

 

[Fire-medic]

Make sure the rubber boots are all fully-seated, and tightened. My bike runs better with the air box on.

The clamps if they are stock have limiters to bring them to a specific tightness. However if you are using generic hose clamps, or if you do NOT have the boots properly-seated, and you tighten them, you WILL build-in massive air leaks, as you seem to have discovered. Generic hose clamps will also tear-up your boots! If for some-reason (they're missing) you are using aftermarket clamps for the carburetor boots and the airbox, look for barrel-bolt clamps (below), as they won't tear-up your boots. Try re-doing the boots, loosen everything up, make sure the carbs are fully-into the VBoost boots, and that the airbox boots are fully-inserted onto the carburetor air horns. Then, like a wheel on your car, try tightening the clamps, not fully, do it 2 or 3 times to get them tightened completely.

Look at my sample, you see the rubber boot is in-contact with a horizontal bar cast-into the VBoost body, along the rubber boot's lowest-edge? One, the bar is on the side, the other it's at the front of the VBoost body. Likewise, ensure that the bottom of the carburetors is inserted fully into the VBoost boots (not-visible in these pics) and that the clamps are tightened properly, straight and correctly, to the limit-point of each clamp.

Also, you want the airbox with the lid on, and the clamps tightened.

 

[Paul S Wilcox]

View attachment 71376 View attachment 71377 Make sure the rubber boots are all fully-seated, and tightened. My bike runs better with the air box on.

The clamps if they are stock have limiters to bring them to a specific tightness. However if you are using generic hose clamps, or if you do NOT have the boots properly-seated, and you tighten them, you WILL build-in massive air leaks, as you seem to have discovered. Generic hose clamps will also tear-up your boots! If for some-reason (they're missing) you are using aftermarket clamps for the carburetor boots and the airbox, look for barrel-bolt clamps (below), as they won't tear-up your boots. Try re-doing the boots, loosen everything up, make sure the carbs are fully-into the VBoost boots, and that the airbox boots are fully-inserted onto the carburetor air horns. Then, like a wheel on your car, try tightening the clamps, not fully, do it 2 or 3 times to get them tightened completely.

Look at my sample, you see the rubber boot is in-contact with a horizontal bar cast-into the VBoost body, along the rubber boot's lowest-edge? One, the bar is on the side, the other it's at the front of the VBoost body. Likewise, ensure that the bottom of the carburetors is inserted fully into the VBoost boots (not-visible in these pics) and that the clamps are tightened properly, straight and correctly, to the limit-point of each clamp.

Also, you want the airbox with the lid on, and the clamps tightened.

View attachment 71378

Will do fire medic. I am using the original clamps. Upon closer inspection, I can see that two of the carbs are not seated properly in their boots. NO WONDER it wouldn't run right. (And to think I was SO sure I did the work perfectly. Very humbling, to be sure.) THANKS! Paul

 

[dannymax]

Thanks, Fire-medic, 85VMAX and dannymax.

When I rebuild the carbs, I strived to keep my workspace cleaner than a surgical suite, I read and re-read the manual and posts, replaced lots of parts with new, etc.

As for boots, they are all new, soft and pliable. And yes, I could see light through all of the jets during the rebuild.

I forgot to mention that when I first started the bike after the carb work, it idled fine, provided the choke was on. That's when I synced the carbs and got 'em all pretty close.

Before I dive back into another carb rebuild, I'm checking the things I can from outside the carbs. So far, I checked the Pilot Air Jet #2 (the one under the diaphragm), the Pilot Air Jet #1 (the one in the "throat" of the carb intake), and blew compressed air through the channel that links the two. I also pulled the air/fuel screws, blew out the passageway, then put the screws with spring, washer and O-ring back in and set them to 3 turns out. Finally, I replaced the slide needles, just because I discovered a new set lying around. And of course, checked the itty bitty O-rings under the CV caps. (By the way, I did put the airbox on, but without the clamps.)

Result? ARRRRGH!!! Now it "idles" at 4,000 RPM even with the idle screw backed all the way off. It also pops and coughs, but it does tolerate a little throttle blipping without dying. It also keeps running (sort of) with half-choke.

Here's the kicker: You were right about the possibility of air leaks -- I sprayed starting fluid on all four carb boots and the revs went up like crazy. I was SO confident that I installed and tightened the new boots correctly. Wow was I wrong!

Any suggestions before I jump into carb rebuild #2?

Thanks much,

Paul

The choke needs to be off for a proper sync.

 

[dannymax]

Sounds like you need another cleaning, like it wasn't thorough-enough.

The carbs cannot be properly-synchronized without the engine running. You can set them up so they should allow the bike to run, but then a running bike needs to have the synch done. Check all your boots from the VBoost and the airbox are properly-seated, and tight, no vacuum leaks.

Could you see through your pilot jets?

Seeing daylight does not guarantee a clean pfj Phillip.

 

[Fire-medic]

Seeing daylight does not guarantee a clean pfj Phillip.

Well, I'm going to defer to the carburetor guru on this one. The passageway is so-small, that if you cannot see light, you know it is obstructed. I use an ultrasonic bath, and then a single strand of stainless steel wire to 'chase' the hole. The wire is about .0125" and a pin vise is a good way to grab a single strand for this purpose. As Frank Zappa might-say, use a bit-of "applied rotation."

 

[Paul S Wilcox]

.
UPDATE Monday 4/27: After examining and tightening the boot clamps and installing the airbox, I was able to eliminate the air leaks entirely. Using the starting fluid test confirmed this.

NOW the engine starts right up with the choke on, but still "idles" at about 3,000 4,000 RPM.

The somewhat good news is that I can now partly release the choke and bring the revs down to about 1,200 RPM without the engine quitting. BUT when the throttle is blipped, revs rise quickly but still "hang" at high RPM before slowly coming back down.

Does any of this sound familiar? I'm still holding back on yanking the carbs and doing another clean/rebuild unless absolutely necessary. Anything else I should consider?

Cheers,

Paul

 

[desert_max]

I would go back and revisit to the nth degree your synchronization.

For Fire-Medic>>

As Frank Zappa might-say, use a bit-of "applied rotation...

A man after my own heart. Both FM and Zappa.

 

[Paul S Wilcox]

Thanks desert_max,
If I can get the thing to idle without the choke I will definitely revisit the sync. Before that, I'll probably have to re-do the bench sync. Perhaps I can do this with the carbs still on the bike? Maybe pull the CV caps and slides so I can see down the bores to the butterflies??

Paul

 

[[email protected]]

Idle can be hanging because of a lean condition/air leak.
Idle can be hanging because of a synch issue.
Start the bike, let it run with choke. Back off the choke and turn up the idle speed if bike wants to die. Just enough to keep it running.
Synch again and see if you can get it to idle down after that.
Then synch again if you get it normal idle range of around 1100RPM.
If idle is still hanging double check for air leaks.
What are the A/F screws set at? around 2.5 turns out from lightly seated is a good starting point.

 

[Fire-medic]

Idle can be hanging because of a lean condition/air leak.
Idle can be hanging because of a synch issue.
Start the bike, let it run with choke. Back off the choke and turn up the idle speed if bike wants to die. Just enough to keep it running.
Synch again and see if you can get it to idle down after that.
Then synch again if you get it normal idle range of around 1100RPM.
If idle is still hanging double check for air leaks.
What are the A/F screws set at? around 2.5 turns out from lightly seated is a good starting point.

Yep, that's exactly what I would do. The idle is definitely affected by the synch. I also inquired about the air bleed screw setting, but he didn't post his settings yet.

desert_max, I saw Frank Zappa a few times over the years, and I always enjoyed the shows. I have a lot of vinyl of The Mothers, and including his first solo album, Hot Rats. I think I even-have his US Congress address about rock & roll lyrics.That's the one where Tipper Gore was one of the founders of the Parents' Resource Music Center (PRMC) group (when Frank Zappa contacted their secretary for information-he was interested to-learn what members there were-the secretary told him, "there are no members, there are only founders.") His famous quip was that allowing PRMC's demands to police rock & roll lyrics was, "...the equivalent of treating dandruff by decapitation." He started speaking to the US Senate by quoting the First Amendment to the US Constitution, then, the quip. Senator Gorton (R-Washington) insults him repeatedly and seems to be on a power trip with his condescending, demeaning behavior. It reminds me of some of what I've seen and heard recently coming from Executive Branch press conferences.

His speech (verbatim transcript to the US Senate):
https://urbigenous.net/library/zappa.html

https://societyofrock.com/in-1985-f...f-congress-instantly-stunned-the-entire-room/

 

[Paul S Wilcox]

Great idea, fire medic and mabdcmb

That makes total sense. I will go through the sync process as you described.

By the way, the a/f screws are currently at 2 ½ turns out.

I Will report back with 48 hrs!

 

[Fire-medic]

I suspect that with the full-elimination of any air leaks, this should solve things. "Full-elimination," sounds like one of those annoying click-bait ads for constipation cures: "do-this, every-morning!"

Get-busy, you've got some sheet to-do.

 

[Paul S Wilcox]

Hey guys,

Update: I eliminated all air leaks successfully.

The good news: The engine ran -- without the choke -- and "idled" at about 2,000 RPM.

The not-so-good news: When I attempted to balance the carbs (#3 and #4 in particular), the revs shoot up to 4 or 5,000 RPM and fluctuated like crazy.

Something must be lurking down in the bowels (and bowls) of the carbs that went wrong upon the last rebuild.

I now have the carbs out of the bike and in my "clean room" for exploratory surgery.

Wish me luck!

Paul

 

[Paul S Wilcox]

Forgot to update you guys. Basically I did everything suggested by you fine folks and the carbs are spot on. Instant start, steady idle, clean power from idle to WFO. Thanks, everyone!

 

[Fire-medic]

Yeah, man, 'that's what I'm talkin' about!'

Perseverance: mostly perspiration, a little inspiration.

So-many of the things complained-about on-here, require a bit of direction, and mucho initiative to execute to-completion. Like the other members who frequently-contribute to assist, we're all-pleased to get the feedback that you were successful.