Carburettors

[Firingonone]

Following with great interest, having had an engine" stutter" at 3500 rpm only when opening the throttle slowly for the last year which is driving me insane on my '75 Strada,apart from balancing carbs repeatedly and all the basics, I have had the jets changed, checked the choke seals are ok as above I am wondering about changing the throttle slides as suggested as a last resort. It starts fine and runs well above 4000rpm What type have people tried/bought and had any success with? mine are marked 7447-50 which I think are standard on the 75Strada.

[Steve Brown]

They do sound like standard. If you think a weaker slide will help then filing the required amount off the front edge of the slide will prove it, one way or the other.
Did you try new needle jets/atomisers yet? They are more likely to wear and cause trouble, cheaper to replace too.

[Firingonone]

Carbs overhauled checked and jets changed professionally.Much improved but still the stutter. Would the slides from Kanguro make things worse or better? If not how much and where should the slide be filed?

[Steve Brown]

Carbs overhauled checked and jets changed professionally.Much improved but still the stutter. Would the slides from Kanguro make things worse or better? If not how much and where should the slide be filed?

Aren't Kanguro slides the same? i.e.50 cutaway? The 50 refers to the height in mm of the cutaway, so a 5mm cutaway. The outer facing edge of the slide is 5mm higher than the inner edge. To weaken things file that outer edge to raise it by say, 0.5mm or 1mm. Even the 0.05mm would make a noticeable difference but I suppose you need to know first if weaker is what you need. Have you experimented with needle position? It's quicker and less irreversible. So your carbs were overhauled professionally? Have you got the specs listed so you know where you're starting from? It's a bit of a ball-ache, really.

[Steve Brown]

Also, if this has started in the last year and you had been ok previously with the old carb set up, is it the ignition letting you down??

[Firingonone]

I have new jets and the ignition thoroughly overhauled. No problems found. I haven't moved the needles . I do always use super unleaded and lead additives (ok money to burn!)maybe I should 'nt. I must admit I can't see why the bike changed unless it's wear and tear. I will try moving the needles first . Incidentally I see kanguros have 40 slides i m not sure if this would weaken the mixture or not

[EVguru]

Please before you start doing anything, download and rear the Dellorto tuning guide, so you have at least some idea how the carburetors work.

[Al B]

Having been chasing a solution for a light throttle (trundlling along in a speed limit sort of light) misfire I read this thread with interest. I'd balanced the carbs and throttle cables carefully, changed idle jets and tweaked the idle screws to the best of my ability and couldn't get rid of it. As soon as the throttle was cracked open the hesitation cleared and the engine ran smoothly, but at steady state it was still there. For the sake of six quid I thought swapping out the rubber discs was worth a go, to be fair they were quite heavily dished but still appeared to be sealing.

Having reset the balance and idle the light throttle performance is transformed, no sign of the hesitant running whatsoever.

Thanks to all who contributed with advice and comments

[Paz2112]

OK so I have been reading this thread with interest. I rebuilt my cards when restoring the bike and replace all seals but I don't remember replacing the rubber at the end of these. With my bike, putting on the choke on the front cylinder carb does increase idle and is necessary to get the bike to start from kickstart, proper operation you would think. However putting the choke on for the rear cylinder does not seem to make any real difference, not to the same extent anyway. Also with the front cylinder choke on the revs increase but with the read cylinder choke on the only increase a slight amount. Now keeping in mind that I am fairly confident that the cars are balanced and opening at the same time, I still have an issue which is this.

I have achieved a good colour tune on the rear cylinder, flickering yellow turning to hard blue when revved and back again, but I have never been able to achieve this on the front carb. I never managed to get a true hard looking blue when at revs. The bike idle is lumpy when cold and she will almost always stall, however when hot (after 15 mins or so of riding) she will idle successfully.

I am using standard airbox with airfilter, good rubber mounts and no obvious other issues.

Any suggestions on where to go now would be greatly appreciated.

[3potjohn]

What idle jets are you running? Are the o rings on the chokes and pilot screws OK?Any muck in the float bowls?
John

[Paz2112]

The idle jets I will need to check. The o-ring on the pilot screws I replaced and the float bowls are clean.

[Paz2112]

OK so I pulled the carbs down and using a magnifying glass I was able to read the very worn markings on the jets and needle and I discovered something odd, possibly?

Front Cylinder Carb : VHBZ 25BS
Choke : 50
Idle : 50
Atomiser : 260k
MainJet : 200
Slide 7447.50
Needle Type : E24


Rear Cylinder Carb : VHBZ 25BS
Choke : 50
Idle : 50
Atomiser : 260k
MainJet : 200
Slide 7447.50
Needle Type : E17


My front carb has a different Needle that the rear carb. I can get a good bunson blue burn from the rear but not from the front.

[mbmm350s]

Hi
Which engine is it? is it tuned? those main jets 200 are huge standard would be 115 for sport or 112 strada .
E17 needle and 260K atomizer was standard for strada before 1979.

Mark

[Paz2112]

1977 350 sport. Would the needles have been the same in both and would this be my issue?

[mbmm350s]

Hi
Maybe having different needles is the issue but there are so many variables
For a 1977 sport with sport cam sport piston standard Airbox and standard exhaust the reference settings are.
VHBZ 25BS
Main jet 115
Idle jet 50
Atomizer 260BD
Float valve 200
Slide 7447-50
needle E17 second notch

The 260BD atomizer is not easy to find but can be created from the 260K by drilling the extra holes
These extra holes lean the initial response to the throttle opening where the needle taper comes into play it's all described in the dell'Orto tuning guide.
Many report changing the idle jet to 45 or 43 improves the off idle response. I am running 48 on a 1978 sport but I have nearly brand new HS carbs.
May be the needle was changed chasing another fault but on Morinis the carbs have the same settings.
The data for the atomizers was published in ATG recently.
Hope that helps
Mark