Topic: 750: Shaving carb slides to richen pilot to needle transition (found on cycle X)

[Garage_guy_chris]

I found this poking around late last night on Cycle X's Tech Tips, In my 3.5 years on here i have never heard of such a mod. Just wondering what other people think. This pretty much sums up my issues of having a lean spot at about 1/8 throttle while crusing 2500 - 3000 rpm. I roll on and i get some knocking or pinging.  I have a spare set of slides out of some junk carbs and a lathe so i might just mess around with this and see what happens. too bad its so labour intensive to make the changes...

Right off Cycle X's Tech Tips    http://www.cyclexchange.net/Engine%20Tech%20Tips%20Page.htm

One of our cheapest ( about the cost of a twelve pack of beer for your friendly local machinist) and easiest carburetor modifications is to cut 0.007" from the flat portion of the slide bottom.  This modification is most beneficial in instances where pod filters have been installed or serious air box modifications have been made.  Refer to " More Carburetor Experiences " below for a technical description of why this is helpful.



Here is a real life story and an experience that has worked for us from time to time.

When cruising down the road your bike is normally running about 2 ½ to 3 ½ thousand rpms. Most of the jetting conversations talk about pilots and needle positions. We feel that is a wide range. Why?  There is a sweet spot that is overlooked when transitioning from pilots to needle position. ”The slide cutaway.” Sometimes with pod filters, drag pipes and other custom products you must have a determination to get the job done. Rather than, “I’ll live with its attitude.”

First, pilots, cutaways, needle jets, needles and main jets work together. They cross over, overlap, and need to be delivering fuel and air as needed.

Oh yea!

The cutaway!

The sweet spot!

Here is what we do in many high performance or aggressive situations. We will disassemble the carb bank and remove the carburetor slides. Then we will remove 7 thousands at a time from the bottom of the slide (On a lathe.) this will richen up the sweet spot (Between the pilot and needle position.), which is normally overlooked. Of coarse the carbs will need to be synchronized again for absolute determination of progress. Sometimes after doing this modification, we have gone back to stock pilot jetting. Here is the problem. This tip takes a lot of work and hours, but no money.

Sounds simple?

Some of these tips have cost us many hours in our service department and we would like to support people having annoying carburetor issues. Hope this helps someday.

** Note: Dynos are used with a 25% braking load to simulate heavier bikes that might ride with a passenger now and then.  So, if you weigh 130lbs and you tune your bike perfectly then pick up a chick that weighs 250lbs your bike will need more fuel. So, if your bike runs great in neutral. But runs like #$%* on the road, give it more fuel.

[dusterdude]

Interesting

[Flyin900]

Can't pick up a 250 lb "Chick" anymore I am to feeble! 

[DaveBarbier]

When cruising down the road your bike is normally running about 2 ½ to 3 ½ thousand rpms. 

Really? More like 6k for me.

[flybox1]

When cruising down the road your bike is normally running about 2 ½ to 3 ½ thousand rpms. 

Really? More like 6k for me.

Exactly!
2.5-3.5k rpm is way too low for cruising speeds.  Your not even close to the beginning of the power band, and youre draining your battery. 
Get your rpms up and see of your problems persist.   I think they wont.

[RevDoc]

2.5-3.5k ! I haven't even shifted out of 1st at that level! I'm with Dave & Flybox on that. Get those rpms up where these bikes were designed to be run and see how sweet they handle.

[Garage_guy_chris]

Well some interesting responses Im still getting a feel for the bike. only a few times i have let it cruise as low as 2500rpm you guys are right down there its pretty unresponsive and lacking power. The more I ride it the more it seems to like cruising usually 3200-3600 and if i want to ride it fast 4000 and over as thats when a roll on results in some instant power. 

Although i will admit regardless of RPM I still get the knock / ping during the roll on from about 1/8 throttle where your typically cruising at. I figure i have to try the needles at the 5th clip pos or mess with a set of slides as suggested. It bothers me a bit but I don't know that its really a problem. Plugs come out Tan and look alright so ?!?!?

Its interesting as no one seems to have heard of this trimming of the slides before ?!?!?

[Retro Rocket]

I've heard of the slide  modifications before but, if your engine is pinging you may have to retard the timing a touch...

[Bailgang]

Cruising down a 2 lane highway then yeah I'm at around 6k rpm but I've got to get there first which means going through town and in some areas here that means anywhere between 25 to 35 mph, I don't know about you guys but I don't buzz my engine at 6k just to be in the engines power band while doing no more than 40 mph so I can see where this mod may be useful. If I had a spare set of slides to play with I'd try it too just to see what the difference would be.

[DaveBarbier]

I'm definitely no authority when it comes to this stuff but I'm typically between 6000-7500 rpm on the highway. Around town I'm usually never under 3500.

For me it's not just to be in the power band but it's also to keep the battery charging as Flybox mentioned. I'm not running open pipes so it's not like it's a problem. The bike feels much better at higher RPMs anyway, these ain't low rumbling Harleys that max out at 5k.

[chewbacca5000]

My 71 has no flat spots anywhere in the powerband from 1k to redline.  I wonder if the slides were cut on that bike?  It is fairly inexpensive so may I will send some slides out to Ken and have the done.

[bochnak]

I've messed around with throttle valves before. It's simple when you are working on a single carb setup, since the valves cost 60ea sometimes.

First off, make sure the problem is at 1/8. Mark your throttle grip/handlebar from 0-full and then divide by 1/8 marks so you can see where the issue is for sure.

Also, a quick test you can perform to see if it is lean: take masking tape and block 80-90% of the carb intake (remove pods or airbox) starting from the top down. This will simulate a smaller cutaway. If problem improves, then you are on to something.

Here is more info on throttle valves, if you look at the graph, at 1/8 or 12.5%, the throttle valve is not very effective at this throttle position.

https://www.howtomotorcyclerepair.com/how-to-tune-throttle-valve-cutaway/

[Scott S]

 Can't you just do the same thing by twisting the right grip? If you want more air past the flat part of the slide, give it some gas. Or am I missing something ?

[Bodi]

Getting the low throttle mixture correct is the big problem with pods.
This low throttle zone is where the cutaway is involved: it reduces vacuum at the main jet to lean the mixture: at slight slide opening without the cutaway the engine vacuum would suck too much gas directly up the main jet.
The cutaway exposes the main jet orifice to less engine vacuum, opening up to the air filter side.
If the transition zone between idle circuit and the main jet runs lean, this procedure of cutting the flat end down slightly is good. If it's rich you can machine a larger cutaway. Of course Keihin has many different cutaways available ... but we can't buy them for production carbs.
At higher openings Bernoulli suction is used to pull fuel up, the needle meters flow in the mid-high zone and the main jet size controls it at WOT. That's why you start tuning at WOT and move down, they all interact somewhat.

[flybox1]

Kick it down a gear.
Keep your rpms above 4000 when just rolling thru town.
Noise wont be an issue, your battery shouldnt be draining, and throttle response is still good.

[bochnak]

In my experience, a lean condition will cause the bike to spit, cough, sneeze (whatever you want to call it) through the intake side of the carbs.

Your symptom is pinging? Have you looked into the ignition system? Are you sure there are no vacuum leaks and cabs are synched?

[dusterdude]

True,back in the days of way lean factory harleys,they did the same spit,kick and cuss

[BPellerine]

the lack of diff slides is one drawback to these carbs when tuning them,if you look at amal carb literature the slide cutaway is a part of the process.seems to be a bit of a crapshoot when tuning for pods,some guys have great luck and some don't .billp

[seanbarney41]

....couple things here...Garage Guy, I think your pinging at low throttle/high load is caused by an incorrect timing advance curve, not anything in the carbs.  Perhaps you have worn out advance springs or maybe try less initial advance or limit total advance to less than 30 degrees.  You need to fix it though.  Audible pinging means bad!

...DaveBarbier, even though it has not been stated, I am pretty sure the original piece from CycleX is referring to 750's ONLY.  Your extensive experience with 550's and their astronomical rpm range is not really relevant.

...Flybox, normal people will ride their 750's at 2500-3000 rpm at or under 25mph.  I will do it on occasion too, it's just that I do not intentionally ride anywhere where I am forced to go that slow.  There are strange phenomena in this world known as farm equipment, traffic jams, road construction detours, and the distinguished gentlemans ride etc.  The world keeps turning and the 750 does just fine.

[Retro Rocket]

....couple things here...Garage Guy, I think your pinging at low throttle/high load is caused by an incorrect timing advance curve, not anything in the carbs.  Perhaps you have worn out advance springs or maybe try less initial advance or limit total advance to less than 30 degrees.  You need to fix it though.  Audible pinging means bad!

^^^This  ^^^

[flybox1]

...Flybox, normal people will ride their 750's at 2500-3000 rpm at or under 25mph.  I will do it on occasion too, it's just that I do not intentionally ride anywhere where I am forced to go that slow.  There are strange phenomena in this world known as farm equipment, traffic jams, road construction detours, and the distinguished gentlemans ride etc.  The world keeps turning and the 750 does just fine.

Sure sb41, situations will call for slower riding, and lower rpms.  It happens to the best of us.   
Just dont, as the OP indicated "cruise' there for very long 

[jonda500]

I have to agree with bailgang and seabarney on this- if I get stuck riding in slow suburban traffic barely hitting 30mph it would seem quite ridiculous to maintain the revs buzzing at 6000 - 7500, I reserve the upper rev range of my bike for when I am holding the throttle MUCH more open than I would be at 25mph (or when an opportunity to overtake is imminent!).
I have never had my battery go flat from 15 - 20 minutes of 2000rpm - 3500rpm riding and I frequently encounter peak hour traffic where higher rpms are really quite unnecessary!
John
n.b. My bike is a 550 with a headlight on/off switch which I turn off in the day time.

[Bailgang]

Guys what's all the hubbub about? All the slide shaving thing is is just another procedure to consider when tuning carbs especially when using pods and it's obviously not a new idea either but it seems to be getting a bad reaction here and I don't understand why, it's just a tip to consider nothing more nothing less. I have my doubts it will solve the OP's problem with pining but it's still a tuning tip worth learning about.

[DaveBarbier]

...DaveBarbier, even though it has not been stated, I am pretty sure the original piece from CycleX is referring to 750's ONLY.  Your minimal experience with 550's and their astronomical rpm range is not really relevant.

Fixed that for you. I'm a greenhorn when it comes to these things!

[seanbarney41]

I also have a hunch that problems in the 1/8 throttle range may often be caused by worn out needles/needle jets and the slide shaving is a rather indirect way of fixing it.