1981 xv750 - No run condition after carb rebuild

[stevedaugherty]

Hello,

I bought a "bargain" xv750 that that a buddy of mine had the misfortune of shearing a forward camshaft and subsequently, damaged the valves.  I spent the better part of last summer ans some of the fall, stripping it down, painting the frame, replacing exhaust, cleaning carbs, wiring repairs....you get the picture I hope :)

I finally got it running in mid fall and got to put about 800+ miles on it (while continuing to work on the numerous other things that needed maintenance or rebuild) until cold weather set in.

I started it a couple of times over the winter....always had a bit of a time getting it started and since there were some "dead" spots over throttle range when riding, I figured carb rebuild should be on the ever growing list....

Of course, it had the hideous grinding start issue.  

So, when spring sprung, I ordered parts to rebuild my starter clutch and related and rebuilt the carbs......Got it all back together, the starter issues is 1000 times better - BUT it runs like absolute kaka and no amount of adjustment to the idle mixture is making things right.

It seems like its WAY to rich;

It wont idle without constant choke / throttle manipulation IF you can get it to start at all.
Constant low level backfires (burps) in the intake plenums and an occasional exhaust backfire.
Plugs fouled coal black.
If you remove the intake boots, you can see residual gas coming back through the intakes.

So, I figure my float level is too high causing some flooding.  And now is where I am stuck,
I've tried to measure my fuel level per the Yamaha manual, but it says you should idle the bike for a few before checking.... I can't keep it running long enough to even contemplate doing that.  I figured if I put the petcock on prime, I should at least be able to get in the ball park....However, it shows the gas level as being low instead of high (about 3 mm down from spec).  

I am at my wits end....At least before I tried to rebuild the carbs I could ride it...Now all I can do is watch it taunt me.  Any pointers would be appreciated.

Thanks,
Steve

[yamaguzzi]

http://www.ccworks.org/portal.php
will set your carb and intake woes straight...one intake one carb,,,major improvement in control,,low and mid range torque/speed....under 100.00,,you may have to switch to a carb with accelerator pump to get that low end juice,,,but everyone ive seen use it loves it.....for your first question on checking float levels,,,a length of clear tobing to fit on the drain at the bottom of the float bowl,,,hold it up above the carb and open up the drain screw,,,watch to see where the level stops,,,,it should stop just below the seam where the bowl meets the carb body....if it is above the seam, check your oil for gas overflow in to the crankcase,,,before you start it again youll want to change the oil and filter and of course rectify the float level....as far as adjusting the idle,,,your carbs have the regular thumbscrew adjustment on the right,,but there is an adjustment on the left side too,,,its in the form of a set-screw with a lock nut it probably needs a slight tweeking to make up for the age and wear of the linkage...now then what about your pilot air screw?  when you rebuilt the carbs did you put new o-rings in ?  this is the screw that is accessed from underneath,,,,its a long brass screw with a spring/a washer and o-ring.....keep in touch yg

[The Alpha]

STarter issue is usually the flywheel teeth being ground down by starter.

Older Viragos had this issue if you were not real careful starting them.  DR Piston knew a guy who used to reverse the gearing on the flywheel to fix this as it is just the first bit that gets chewed up.  

If you must run the choke the bike is running way too lean as the choke richens it up and keeps it running.  But in doing so it is not the correct ratio.  

Check your carb to head seals.  (Spray some WD40 while it is running around the boot, if the rpm changes you found the leak).

Check to insure all vacuum lines are on and in good shape.  Putting the pet cock to prime floods gas into the engine to prime it.  That will flood the motor if you do it more than a few seconds.

The problem is your carbs.  If there is no vacuum leak then you will need to pull them and figure out what you did wrong to cause it to run so lean.

[stevedaugherty]

Hey folks,  Thanks for the reply's :)

I checked the fuel level and found them low...  Adjusted them and they are both now about 2 mm under the seam.  

Its running, but still not right.....I've tried altering the fuel/air mix screws and am still getting lots of low level backfires and its still fouling my plugs out.

I did replace the orings on the mix screws.

Another "weird" is the rpms take awhile to come back to idle.  For instance, I turned around after running down the road a bit and when I clutched in, the RPM's stayed at 4000 for about 5 seconds or so before coming back down.  

I tried the wd 40 test and didnt find any leaks.

I have removed the "fresh air flapper" from the forward cylinder so it's pretty cut and dry on the vacuum line side.  

It still just acts like there is way too much fuel in the mix.  I've altered the mix to lean as can run and still getting fouling.

When I set the fuel level, I looked over my rebuild work and didn't find anything out of the ordinary.  

Could this be caused by a worn needle (for main nozzle) condition?  They weren't included in my rebuild kits  and mine had some "tarnish" build up that I buffed off using a dremel with a buffing wheel.  

Any guesses??

Thanks,

Steve

[stevedaugherty]

Success!!! :)

Cleaned the plugs (again), set the mix screws both to 3 turns out.

Started.

Had to tweak both a bit leaner about a quarter trun, rode, repeat another quarter on the front...

Bingo.

Havent tried highway speeds yet and fear that I might have to raise the fuel level just a bit more, BUT ITS RUNNING RIGHT.

BWAAAAHAAHAAAHA

Thanks for the pointers

[stevedaugherty]

BOGUS !! :(

Fouled the plugs again.  Pitch black in about 25 miles.

Pulled 'em again and found a pinhole in the rear diaphram and the main needle on my forward is nicked a bit.  A search for diaphram replacements has me in sticker shock.

Besides a bullet in the gas tank - Any body have a $ guesstimate for an economical carb solution?

If its too much, the cost of a bullet might win :)

Regards,

Steve Daugherty

[yamaguzzi]

http://www.geocities.com/mike33div/My_Bike_Page.html
look this guy up and give him a call,,keep in touch..yg

[paul escott]

G'day Steve and other lurkers, greetings from Brisbane.

I found this thread whilst trying to figure out the '81 XV750 I've bought as a poor runner. I think I've sorted it but can't check as I'm waiting for the starter recon pack from 'Chopper Bob' in New York. He sells his kit around the world via ebay and it appears on paper to be the dogs bollocks. http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&rd=1&item=250252326677&ssPageName=STRK:MEWN:IT&ih=015
Anyway, back to your carb problem. The way to fix your diaphram is buy a tube of rubber glue, the sort that comes in a pushbike inner tube repair kit, and use that to fix a patch. Now; please don't laugh. You fashion the patch from a condom. Cut two 10mm circular patchs from an unrolled condom, rinse the lube off them with petrol and once dry fix your diaphram top and bottom with the contact adhesive. When i was a youth working in a m'bike repair garage we used to use the Black Trojan brand because we thought that if we used a standard pink Durex the customer would realise what we had done
let me know how you get on.
Regards
Paul

[yamaguzzi]

brilliant paul! its the tricks that define the amateur from the pro. yg

[stevedaugherty]

OUTSTANDING FIX -  I tried some auto grade silicone but couldn't get a bond that I thought would hold.  

I gave up and ordered an old set of carbs off ebay hoping to get usable diaphrams.  Got lucky - both are in good shape and the needles are in better shape than mine.  As luck would have it, the old carbs arrived today along with your suggestion.  But I sincerely thank you for sharing the fix. :)

While waiting, I continued reading up on these carbs.  I found some info that led to another discovery.  The "hidden" main and pilot air jets located under the choke tube.  When I "found" mine, YIKES they, and the area around them are so dirty that I wouldn't be surprised if that is a big part of my problems.

They dont even show on the parts diagrams I have found, but they look replaceable if all else fails.  I'm going to try to clean them and see what happens.

Wish me luck folks and best wishes to those in the land down under.

Steve

[yamaguzzi]

congrats: yes steve the old parts actually dont wear out and can be cleaned and reused..the use of a guitar string may be beneficial in the cleaning of the smallest passages,,also the holes in the float bowl are vents that must be able to breathe..take care yg

[stevedaugherty]

Allright folks,  here's the latest....

After some more extensive cleaning and part swapping (diaphrams, main needles, choke stoppers) I reassembled everything last night.

Idle is improved greatly :) but...

It's still "dead" on the throttle at about 4k rpm (around 55 mph).  It starts missing bad and the speed falls off until you let it fall under 4k.  If you wide open the throttle when its "chugging" there is no effect until you let the engine get under the 4k range.  If you "granny" drive, its fine until you hit the 4k mark.  Also some "chug" if you try a fast run through the gears.  

It seems like once you hit the 4k range, the mix is just too rich.

To try to isolate, I "t"d my fuel feeds to both carbs from one petcock instead of 1 to 1.  No change.  Switched to the other petcock, no change.  Fuel level is good and leaving it in "prime" vs. normal or reserve has no effect.

I pulled plug wires and redid both the coil and plug ends.  No change.

I pulled the air filter it started to run REALLY bad.  I could barely limp it back home.

So, the more air it gets the worse it runs.  I tried to enrich the mix more by backing the mix screws out extremely far and it moved the "chug" range up to around 63 mph.

The pipes are stock but the air filter is a full foam element rather than the oem paper.  Could the difference in differential pressure between the oem and foam element be causing me this much grief and if so, why is it so much more pronounced on the higher range rather than throughout the entire throttle band.

Help o moto guru's.  

[yamaguzzi]

yes the new airfilter is going to make a difference,,probably letting in more air....a couple of things: are your floats adjusted? second your fuel cap is venting properly?..your diaphrams dont have any pin holes? your carb vents clear..ive also heard some guys putting a small #4 washer or two under the needle from inside the piston located inside the carb,..give it a go,,keep us posted yg

[stevedaugherty]

It is def. starving for air on the top end...

I pulled the filter again and used foil tape to block off all except about a quarter moon sized hole.  It was trying to die as soon as I tried to take off so I used the highly scientific method of using my finger to tear the opening a bit bigger.  Since I hadn't adjusted the idle mix, low end was chunky but once I got on the road I was running 70 mph plus with smooth mid to high range.  Full throttle without chug :)

Using another highly scientific method :)  I used my hand to block more of the opening and the chug returned.  

re: Floats adjusted - Yep - after taking them on and off 5 times they are :)  (Should have replaced the bowl screws with allen heads doh)

re: fuel cap vent - Eliminated that awhile ago by opening the fuel cap while the chug was going on.  

re: Diaphrams - Got old replacements and verified that they were 100% before installing.

re: Carb vents - I'm assuming that your refering to the pipe on the top end over the diaphrams - Yep 100% open and the small filter elements are unrestricted.

re: Washer under the needle - Would that add fuel or air???

Right now, my plan is to get an oem air filter since the rebuild kit I got for my carbs had factory spec jets.  Hopefully I can dial the the idle mix and still have enough air for the top end.  Otherwise, I'll probably foil tape some wimpy reticulated foam over the opening and add layers until I find some kind of low to high end balance.  Unless your washer suggestion will increase available air.

Wish me luck and thanks for your continued help!!!!

SteveD

[stevedaugherty]

New filter got here today.

Since I had to wait a few days for it to arrive, I snipped 1/4" off each plug wire end to freshen the ends, found and replaced a loose fitting vacuum hose on the forward petcock feed, took apart my TCI (ignitor box) and touched up (resoldered) the primary in/out wiring connections.  

I set the idle mix screws to 3 out from soft seat and tuned in the front to 1 and 1/4 out and rear to 5 1/2 out from soft seat.  52 miles of problem free driving with smooth(er) idle and full throttle range without "chugging".  Still not perfect but I figure the plugs are probably pretty dogged out after fouling out and being cleaned about 15 times :).  

It's close enough that at least I can ride while I finish debugging.

Sincerest thanks to all those who posted tips, tricks and hard earned knowledge.

Best of luck,
Steve

[carlosb.7769]

synchronize carbs when you remove them. becareful not to damege your manifolds upon removing the carbs as they are hard to find and/or replace. from my experience the right carb air screw should come out approx. 3 11/12 turns after bottoming and the left side approx. 4 3/12 turns after bottoming.