668 carb bog

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R

rampage idiot

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Anyone know if there is any tricks to lessen the initial bog off the line using a 668 carb, I know this is a none problem just wondering if anyone has come up with any thing
 
bog

i have had thesame trouble- always on 2 bolt engines- put the same carb on a newer 4 bolt- runs perfect. gotta wonder if this has to do with port timing piston skirt cut away? - These engines should have timeing marks- only engine -so far that told the spark timing was a zen engine- every time the coil is removed- re attached -it has to not land on the same place- This is just a thought-- who knows?
 
Yea gts, this makes me wonder if this is an ignition issue- This photo is a step in the right direction- if ya gat a big wallet-$$, it still has a fixed timing- the british have ignition modules that can be adjusted tru programming- electronic- just like the cpu on a modern full sixed car- lorry.---
 
Thing is about importing these parts- its the euro-dollar ratio. Not wishing any thing bad for Europe- but from my crystal ball view, this ratio could be changing soon- good for America- bad for Europe. I had a expansion chamber shipped state side from Scotland last week- Tru British royal mail- it cost 40 usd- was here in 6 days-
 
I have drilled out the main jet passage and pressed in a proper main jet ,you can purchase them from a walboro parts list .better know what you are doing though when reaming the hole .the way I see it the carbs are garbage anyway with a fixed main jet.Roger
 
walbro

the main you replaced- is this the bronze jet- check valve feeding into the carbs bore-? this has to be.- I bought a 813 -thought the pump outlet would have its own seprate outlet- pump the fuel out as a fine mist like a holly- or barry grant would. wet the carb down before installing on the engine, kept moving the throttle until the pump primed up- eventually, it loaded up- this pump feeds the fuel out of the main-check valve. there is no mist-stream of fuel- just about 1 drop per pump. This carb was brand new- being mass produced - does not mean its correct- But to feed the pumped fuel tru the darned main -check valve is micky mouse.- just my opinion.
 
RAMJET said:
I have drilled out the main jet passage and pressed in a proper main jet ,you can purchase them from a walboro parts list .better know what you are doing though when reaming the hole .the way I see it the carbs are garbage anyway with a fixed main jet.Roger
Click to expand...
yea ramjet- i see all the people cutting -streching metering springs on other sites- even the ultra-lite airplane guys do this- saying the sweet spot is 12-15 psi reset. trouble is- thats just to close to the fuel pump pressure- its just gonna go rich right off the bat- yes, it may seem to really make the engines run stronger- but this is not corrrect- to see people do this on a airplane frightens me- ya cut springs, its lost its squareness, the metering spring could easly slip out of the indentation that keeps it in place on the metering lever- - walbro does have the carbs with a slide in the bore, instead of the butterfly- but the way the jets adjustment is designed- its a true "johnson rod " set up if i ever saw one- Shame Mikuni does not make something like the wt series carbs- these could be tuned with threaded fixed jets - air bleeds.
 
yeah the 668 carb just has a hole drilled for the main jet , the better carbs have a valve pressed into the main jet cavity ( it only opens when there is adequate pressure differential ) the 668 just slobbers fuel from the main jet whenever there is low pressure present at the main jet .the better carbs shut the fuel off through the main jet until a predetermined pressure differential is reached .this gives the carb better transition . I agree I would not mess with anything I was going to sit in to fly .but r/c cars and giant scale planes ,why not, especially when they come with a useless stock carb like the 668,if your good with a drill and reamer for $5 you can make a real carb from the 668. Roger
 
bog city in hot weather.

not certan what drill. O.K.,, lets figure this bogging issue out together site readers. These are my symptons- my engines are leak free- tighter than my ex-bosses as$. For me- the 813 was the biggest bogger of them all.Yet, this carb has a fuel enrichment circut- accelator pump. The hotter the air is, the richer the engines tune goes- opposite for cold. No matter what the actual air tempeture is, a gasoline engine will be somewhat lean @ start up. As the engine comes up to running temp- it runs richer. I fire up the motor, set the lsn half way between the lean-rich drop off point- the engine is not quite up to temp- as i said - hot engines go richer.- Drive the car for about 60-120 seconds, no bog.- drive the car and start hammering the throttle- Its up to running temp now-- and lo and behold- mr. bog has reared its ugly head.- I whip my----- screwdriver out, and richen the lsn. thes lessens the bog, but its no longer in its ideal mix @ idle- or just above idle- you know the drill folks- raise the idle rpm via the butterfly adjustment. All these things point to an engine that is not a lean bog, but a rich bog. Not limited to the 813 either-668-to a lesser extent, the venrable 990 also. For me-- this bog lessens in cold weather- 30-4-5-60 degrees.-Further galvanizing that its a rich bog. I can make a 990 run like a bat out of hell in the winter- pressure spring, small notch on the butterfly plate srraddling the lo speed ofirce-bleed holes.- no bog- the engines best settings on the lsn is halfway between the lsn-s rich-lean drop off point.-In hot weather?- its plug foul city- wayy-to much fuel.- I go back to the stock pressure spring.- I really think that these carbs just need a larger air bleed- on the low speed circut.- 1 or more holes - not the first hole on the air intake side of the carb-you know, the 3 holes in the bore-- but 1 or more of the rear 2 holes.- these holes operate in 2 modes-3 stages depending on the vacume- rpm-butterfly opening.- Cripe- i am outta breath- chough-chough-- I did order a 771 a few hours ago- gitmotor= good deal! - What are your thoughts on this wreched bog???
 
just want to add- do not want to come off as a know it all- i am far-far fron that title- I could very well be completely off base about this bog issue also- with hundreds of site members out there, if wee pool our thoughts we might be able to get a handle on these bog issues.- just saying---
 
I found the best way to remove the bog in my FG Marder was to install an 813 and a 9500rpm clutch spring. The 813 eliminated the bog almost completely, but the higher spring helped alot as well. But this was in my 20 pound 2 wheel drive buggy with skinny spiked tires. This may not be the solution for a 40 pound Rampage M/T.

I lean my ls needle in until I get the highest rpm then back it off 1/16 to 1/8, if I have any bogging, I'll back it out some more. I run my carbs a bit rich so I can hear a nice 2-stroke pop, if it's leaned to far it becomes a dull nagging sound that drives me nuts... Basically the sound of about 80% of YT vids.

I tune the hs the same way, rev her to the moon and lean it in to find the highest rpm, then back it out an 1/8th and call it a day. My plugs are normally grayish black, never brown. I still have the same plug after 15 gallons, so I guess I must be doing something right.
 
yea, these engines seem to produce better power if they are a little on the rich side. more margin for tuneing errors also. Thats what i did with the 813 equipped engines too- higher clutch rpm lock up. On slick surfaces and smaller running areas with a big bore engine a lower rpm engagement-lock up the lower rpm spring helps me control better. I did hit the 990s sweet spot big time- cardboard color-little bit darker- at idle -midrange-top end.- Jetpro rear-ada short manafold- 3/4" velocity stack.- I slowly filed a notch in the butterfly valve- I did eventually go to far with the notch- just a slightly high idle even with the butterfly valve closed. This was a sweet running carb- added power too with zero bog.-I did place an order tru gitzmtor yesterday-have a few butterfly valves-771 on the way.- I saved and marked the above mentioned butttterfly valve with too large of a notch. this will be a reference for the next notch mod.- Need just enough to be able to adjust the idle speed. The notch acts as a velocity stack across the 3 drilled holes you see in the carbs bore- stronger signal. Say, did good this time on spelling-only bout 10 mistakes!! lol!
 
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