VHB29 and 750 Ambo Help. Solved I think.

[Tom H]

The problem: The bike had a flat spot at full throttle. Go to WO on a freeway at about 65-70mph in 4th and it would bog and not accelerate. If I let off a bit, it would go, WO again and bog then maybe go. When I say it bogged, I don't mean for a second, I have to let off or it wouldn't go, actually feels like it's slowing down. It had been like this as long as I can remember. Have had the bike like 30 something years. Don't remember when it started or if it's been this way the whole time. My Eldo was my main bike, this was my ride when the Eldo was down kinda thing.

The first fix attempt: New stock size needles. Didn't help.

The fix: I changed the main jets to a 140 instead of the stock 145. This got rid of the bog and the bike ran well. I changed the jets during the winter, so temps were about 60-65 deg.. All seemed well.

The new issue: I have not ridden the bike in hot weather until the last week, it has been ridden frequently, just not in hot weather. It's now about 85 deg. this past week, had been in the 70's. I took a ride that I have done often that is mainly freeway. This time the bike started pinging when I got on the gas at about 70mph. Going up a hill it would ping. Just cruising along, it was fine.

The history of what I have done recently about the time the hot weather started: I rebuilt a working distributor, shimmed the shaft, new condenser, points and advance springs (1ea. 8 and 9 coil. Orig had one spring with a long/oval end loop that didn't seem to hold the weights back at all), same cap, rotor and plugs. Set the point gap on the bench and installed the distributor. Set the timing with a light. Sounded great!

Went for my first ride since changing the distributor. Took the same ride as above (good test type ride, freeway and streets). The weather is about 80. The bike did the ping at about 70 when passing a car and going up a hill. Before I headed back, I took a little timing out, no light, just retarded it a bit. Seemed to help on my trip back.

Got home and checked the timing, about half way from TDC to first mark. Checked the points and found they were slightly tight. Adjusted to 17 thousandths again and reset the timing to the first mark from TDC.

Today I tried the bike again on the same route. Still pinged with the hot weather, almost 90 today. Got back and changed the main jets back to 145's. Went on the route again. No ping... Yeah!! But...back to the flat spot issue at full throttle.

The question: Where do I go from here? 140 mains, no flat spot but pings. Stock 145, no ping but flat spot. Would staying with the 145's and moving the needle down one help this? Going back to 140's and moving the needle up one? Or.....???

Thank you!!!
Tom

[guzzisteve]

Walk a fine line & put in #142's or 3's.     850T's came w/142's stock w/new.

[Antietam Classic Cycle]

I used to have the same issue when I first got my '69 Ambo. Never could completely eliminate it, until... I bought a set of original "big hole" cigar mufflers at a swap meet and put those on in place of the "universal" mufflers that it came with. Totally transformed the bike - flat spot gone, pulled much better than before. The "ping" stuck around until I did a top-end rebuild including Gilardonis and rebuilt heads.

[Tom H]

Interesting.

The mufflers are cigar small hole, the ones that MG Cycle had that were repros of the small hole. I tried a set of MAC without the crossover from my 850 (850 is happy with them) and the 750 didn't like them. I still have my standard/large hole cigars from my 850, but there are in bad shape. Been so long since the change on the 850, but I think it's happier with the MAC's.

I did change to the Gilardoni set. But I have not had the heads done. I need to find someone that can do a complete head job without cutting the valve seat at an angle because they installed the guide at an angle.

Maybe the 143ish jet might do it. What about raising or lowering the needle?

Again, cooler weather I was good at 140 mains, hot weather not so good. Heck, if I wasn't so nit picky and trying to get it to run perfect, someone else might say it's fine on the streets at 50-60mph. If I have to pick one or the other, I'll take the bog at WO.

Thank you!!
Tom

[Tom H]

On the subject of mufflers. I have a set of MAC Cone end style for loops. Let's say I try them rather than the slash cut from my 850. Where should I go with the main jet? 150? 135?

Thanks again for your help so far!!!
Tom

[John A]

A bad air filter will drive you nuts. Seems especially true for an Ambo. Don't ask

[Idontwantapickle]

Tom,

Have you checked the timing at full advance? The low speed setting is fun and all but the really important stuff happens at full throttle at high engine speed. Check the full advance, the info for timing marks vs engine speed is in the service manual IIRC.

Hunter

[reidy]

Tom

Here is a left field idea for you. It appears the bike was running rich so you reduced the jet size. Is there a possibility that the o rings in the enrichment circuit have perished and therefore giving the effect of having the choke on all of the time? This will make tuning difficult as know one tunes for performance with the choke on. It causes most of the logic we use to go out the window.

Steve

[Tom H]

Hunter, When I timed it without a tach, I gave it a good amount of revs and the advance topped out at the full advance mark. Might have to figure out how to hook up a tach.

Steve, New O rings on the choke tubes as well as new choke slides. Also, the cables are loose and not hung up. I have never needed the "choke".

John, Uni filters, not clogged.

Thanks again so far!!
Tom

[Idontwantapickle]

Hunter, When I timed it without a tach, I gave it a good amount of revs and the advance topped out at the full advance mark. Might have to figure out how to hook up a tach.

Steve, New O rings on the choke tubes as well as new choke slides. Also, the cables are loose and not hung up. I have never needed the "choke".

John, Uni filters, not clogged.

Thanks again so far!!
Tom

Tom

That's good, Going to full advance and not beyond helps rule that out as a culprit.
Given that pinging is a much more serious issue than bogging at full throttle I would recommend jetting it to keep the ping away and tolerating the bog. There may be other issues, like worn valve guides, but for now to keep it on the road by eliminating the pinging.

Hunter

[Tom H]

That is what I was thinking.

After reading the Dellorto tuning guide for the umpteenth time, It looks like the needle does not affect WO throttle, only the main jet. Did I read this right?? OR...Could lowering the needle one notch help my WO issue??

Thanks again!!
Tom

[John A]

that still sounds like an air filter problem. try it without the filters to see

[Tom H]

Well we might just have a winner.

Moved the needle down one notch. Hit my test route. All seemed well. No WO flat spot, well a very, very slight one. Now I listening for ping, all good so far, until about 5 min down the road I thought I heard a ping. Got to a hill and the ping was back. The one notch dwon leaned it out like the 140 main.

Pulled to a side street with some shade. Changed the needles back to the middle notch. And pulled the air filters for the trip home.......

No ping and no flat spot!!! At least so far.

John A. is the winner!!! What does he get? A BIG THANK YOU!!!

Now I need to see about a new set of filters. I didn't think they were dirty. I guess that's what I get for thinking. I'll see how it does with new filters, hopefully it stays running great!!

Thank you all again!
Tom

[guzziownr]

You can put unlimited fuel through those 29s but you can only increase the air by so much.
 

[John A]

cool, like I said a bad filter will bite you in the butt occasionally especially on those early engines. when I discovered it I did all the stuff you did so finally I thought I would feel the intake air for a side difference. it was about 50F and with out the filter my engine was back!
OK so then at wideFopen the air at the carb intake was puffing back and forth and was warm air.

[Antietam Classic Cycle]

[Tom H]

Charlie, I still have that style for both bikes IIRR, I'm sure I have one. I always hated trying to get the rubber to seal to the box. Is there a better rubber for them now that holds the shape, or a metal ring to secure the rubber to the box, or......????

I prefer the pod style so I don't have to mess with the box for service. But at the same time, I have always wanted a decent filter. I don't mind a paper filter. I did have K&N on both until the rubber hardened and couldn't fit them properly. Then went to Uni.

John, Go figure. The filter was the last thing on my mind for the WO issue.

Charlie, I used your tutorial for the distributor rebuild. Great info!  It puzzles me as to why they would use .2mm or so shims, but still leave such a huge gap??? I shimmed one with 1.2mm of shims, still had about .009"/.230mm clearance after shimming.

Thanks all again!
Tom

[Antietam Classic Cycle]

Charlie, I still have that style for both bikes IIRR, I'm sure I have one. I always hated trying to get the rubber to seal to the box. Is there a better rubber for them now that holds the shape, or a metal ring to secure the rubber to the box, or......????

Same old rubber as always. I always do a "dry fit" first: install the boot on the carbs, positioning the boot so the sealing surface is vetical, flat and not distorted. Then test fit the box and determine the position where it fits best (top tab to the left of the tab on the frame, tab to the right, etc.). Once you're satisfied with the fit, apply a bead of Permatex Ultra Black RTV to the sealing surface of the boot, hang the 'box from the top bolt and swing it down onto the boot, install the bottom bolts and let the RTV cure.

The boot will stay "glued" to the cover of the 'box, so I just pull the bars w/manifolds and remove the two bolts securing the plate to the 'box when it's time to check/change the air filter.